Year 11 Physics Momentum and Kinetic Energy

A question asked by students in Year 11 Physics  is "what is the difference between momentum and kinetic energy?" Moving objects have both momentum and kinetic energy but each quantity plays a different role in Physics. A good description of why we need both momentum and kinetic energy is given by Serway and Jewett in Physics for Scientists and Engineers (eighth edition page 236)......There are clear differences between kinetic energy and momentum. First, kinetic energy is a scalar and momentum is a vector. Consider a system of two equal masses heading towards each other on a line with equal speeds. There is kinetic energy associated with this system because members of the system are moving. Because of the vector nature of momentum however, the momentum of this system is zero. A second major difference is  that kinetic energy can transform to other types of energy, such as potential energy or internal energy. There is only one type of linear momentum, so we see no such transformations when using a momentum approach to a problem...

Momentum can be thought of as the tendency of an object to keep moving in a straight line at the same speed. Kinetic energy is  the work done (energy provided) by an external force in moving the object from rest to its final speed.

From a mathematical perspective, in classical physics momentum is defined as mass multiplied by velocity, p=mv and kinetic energy is given by E_k=1/2mv². Notice that If we differentiate 1/2mv² with respect to v we get mv which is of course the momentum. This generalisation is one of the first steps in the long road to the development of quantum mechanics. A tutorial sheet on momentum and kinetic energy follows.

1. Express E_k in terms of p and m.
2. Two objects P and Q, have masses in the ratio of 2:1 respectively. If each has the same momentum which has the greater kinetic energy?
3. Two cars are moving along a road. The mass of one car is twice that of the other but it is moving at half the speed of the smaller car. What is the ratio of the kinetic energy of the larger car to that of the smaller car?
4. Two objects A and B are in motion. The kinetic energy of A is one quarter that of B and the momentum of B is one half that of A. What is the ratio of the speed of A to the speed of B?
5. A trolley of mass 452g is moving in a straight line on a smooth horizontal laboratory bench. A block of plasticine of mass 146g is initially at rest. Determine the change in kinetic energy of the system when (i) the block is on the bench and the trolley collides with and sticks to the block (ii) the block is dumped on the trolley from a small height as the trolley passes underneath. Describe what has happened in each case to the missing kinetic energy.
6. A large mass M is at rest on a smooth horizontal table. A smaller mass m moving at a velocity u collides with the larger mass. If the collision is perfectly elastic determine the velocity of the smaller mass after the collision.
7. In the previous question initially the larger mass M is moving at a velocity U and the smaller mass m is at rest. Determine the velocity of m after the collision.
8. A trolley of mass M contains a mass m of sand. The loaded trolley moves at a velocity U along a smooth horizontal laboratory bench. The sand starts to leak from the trolley at a constant rate R. Determine the velocity of the trolley when one half of the sand has leaked out.
9. *Three perfectly elastic spheres of masses m₁,m₂ and m₃ lie in that order and not in contact in a smooth horizontal groove. If m₁ is projected towards m₂ with velocity U find the velocities of each sphere after two impacts have occurred and show that there will not be a third if m₂(m₁+m₂+m₃)>3m₁m₃

HSC Physics Space

With the old Space topic being examined for the last time this November a list of true-false questions on concepts that are often misunderstood is given below. This list will be updated during the year.

Gravity

1. A satellite has a greater gravitational potential energy than a grain of dust in the same circular orbit about the Earth.
2. Gravitational potential energy is the energy needed to bring masses together from a state where they are not influencing each other.
3. A gravitational sling-shot of a spacecraft by Jupiter causes the spacecraft to leave Jupiter at a greater speed. [false, spacecraft can increase speed relative to the Sun by "taking" kinetic energy from Jupiter in an interaction in which the spacecraft has no change in kinetic energy in Jupiter's reference frame but an increase in KE in the Sun's reference frame due to Jupiter slowing down and giving KE to the spacecraft. The final spacecraft speed can be increased by almost twice the speed of Jupiter if the spacecraft moves directly towards the approaching planet whose gravitational field sweeps it around increasing its speed relative to the Sun]
4. Two large equal masses M are placed a distance r apart. A smaller mass m is placed at the midpoint of the line joining the larger masses. The gravitational potential energy of this system is zero.
5. Two large masses M are placed a distance r apart. The work done in moving a mass m from a very large distance to the midpoint of the line joining the larger masses depends on the path taken.
6. Gravitational potential energy increases as a mass moves closer to the Earth since the gravitational force increases.
7. A mass is tied to a string of length 120 cm and moves freely in a vertical circle in the Earth's gravitational field. The speed of the mass at its lowest point is 8.0 ms ^-1 . The magnitude of the acceleration of the mass at this instant is 53.3 ms ^-2 .[false,54.2 ms ^-2 ]

Projectiles

1. A projectile moving upwards has a negative acceleration and when it moves downwards its acceleration is positive.
2. A cannon is fired horizontally from a tall mountain. The projectile can strike the Earth on the hemisphere opposite to the direction of firing if its initial speed is sufficient.
3. An object is dropped from the Eiffel Tower. Neglecting air resistance the object hits the ground at a point to the east and south of its starting point. An object dropped from Centre Point Tower (neglecting air resistance) will be deflected to the west and north of its starting point.
4. A ball is thrown at initial speed U on horizontal ground. The maximum range of the ball is R. If the new initial speed is 2U the maximum range (neglecting air resistance) is 2R.

Orbits

1. The two forces acting on a satellite moving in a circular path around a planet at a constant speed with no air drag are the gravitational force and the centripetal force.
2. When atmospheric drag acts on a satellite its speed decreases.
3. Apollo 13 could re-enter the Earth's atmosphere at an angle 𝜽 to the vertical where 5.3° < 𝜽 < 7.7°.
4. A spacecraft that bounces off the atmosphere enters an orbit around the Sun.
5. A satellite is in a low Earth circular orbit. The radius of the orbit decreases. The gain in orbital kinetic energy of the satellite is equal to the loss in gravitational potential energy of the satellite.
6. A satellite in a high Earth circular orbit has a total energy E. If the satellite is placed in a circular orbit of twice the radius its total energy is 2E.
7. A satellite in a circular orbit of period 23h 56m 4s that passes over Sydney always appears directly overhead.

Relativity

1. An electron cannot move through water at a speed greater than the speed of light in water.
2. A rocket moves at a constant velocity of 0.90c relative to the Earth. The crew of the rocket play a CD that lasts for 1 hour. A person on the Earth plays an identical CD that lasts for a longer time interval than one hour.
3. A particle accelerator is 2.0 km long. A proton starts from rest and moves through the accelerator striking the end at 0.8c. The distance travelled by the proton in its own reference frame is 1.2km.
4. A certain star is 66.5 light-years away from the Earth. A spacecraft leaves the Earth and travels at a constant speed of 0.95c to the star. The time taken to travel to the star according to a clock on the spacecraft is 70 years.
5. A train is moving to the east. A ball is rolled across the smooth floor of the train initially perpendicular to the south side of the train at 1 ms ^-1 .A person in the train sees the ball move in a parabolic arc towards the east of focal length 1 m. The acceleration of the train is 0.5 ms ^-2 to the east.
6. A white hot metal rod is cooled to room temperature.Its mass does not change.
7. In Michelson and Morley's interferometer the light rays interfere destructively. This is called a null result.
8. In the aether theory the time taken by light to travel along each of the equal arms of the interferometer is the same.
9. Interference fringes are not caused by the reflections from the half silvered mirror in the Michelson-Morley experiment.

10. The result of the Michelson-Morley experiment is that the time taken by light to travel a given path depends on the direction of the light ray.

11. Simultaneous events are seen to happen at the same time instant in the same reference frame.

12. Michelson and Morley measured a change in fringe spacing when they rotated their apparatus through 90°
13. Michelson and Morley expected to measure a change in fringe spacing when they rotated their apparatus through 90°

Year 11 Physics Vectors Sheet 1

This is a sheet of harder questions on vectors.

1. A river flows at 3.0 km/h. The river is 100.0 m wide and a boat is to arrive on the opposite bank 80.0 m downstream from its starting point. What is the velocity of the boat relative to the water if the time of crossing is 5.0 minutes? [2.4km/h at 31° to the upstream bank]
2. A river 1.0 km wide flows due north at 8.0 km/h. A motor launch travels at 6.0 km/h relative to the water. A person starts from the west bank and wishes to reach the point directly opposte on the east bank. If the person can walk at 3.0 km/h find the direction in which the motor launch should head so that the person can make the journey in the minimum time.[E33°S]
3. Relative to a cyclist travelling to the west at 40km/h the wind appears to be coming from the south. On doubling their speed the wind appears to be coming from the south-west. What is the velocity of the wind?[56.6km/h NW]
4. A cyclist travelling on a straight road at 10 km/h is subject to air resistance proportional to the square of their speed. By what factor must they increase their power output to maintain their speed if a 20 km/h cross wind develops?[5^1/2]
5. An ocean liner is travelling in a straight line at a speed of 20km/h that takes it 1.0 km from a port. A boat that can travel at 12km/h is to leave the port at the last possible time and move in a straight line to meet the ocean liner. What is the distance travelled by the boat when it reaches the liner? [1.25km]

IB Physics Mass and Spring

A tutorial sheet of questions on a mass and a spring is given below.

1. Two springs each of force constant k are connected in series and held vertically. A mass M is placed on the lower end of the combination. What is the extension produced?
2. A mass M rests on a smooth horizontal surface. Identical springs of force constant k are attached to opposite sides of the mass. The other ends of the springs are held at rest. What is the period of oscillation of the mass when it is released after being displaced (a) along the line of the springs, (b) perpendicular to the line of the springs.
3. A spring of force constant k is held vertically with its upper end fixed. A mass M is placed on the lower end of the spring. The mass is then pulled down a distance A and released from rest. (a) Explain why the period of oscillation does not depend on g. (b) Does the period of oscillation does depend on A? (c) Instead of being released from rest the mass is given an initial velocity u downwards at the pulled down position. What is the period of the oscillation in this case?
4. Does a mass oscillating on a spring in a vertical plane have gravitational potential energy?
5. A light spring is hanging loosely from the ceiling. A mass is placed on the free end of the spring and released from rest. The mass moves downwards a distance of 40 cm before starting to rise again. What is the period of the simple harmonic motion?[0.90s]
6. A block of mass M is connected to a light spring of force constant k. The block is placed on a smooth inclined plane of angle of elevation 𝜽. The other end of the spring is held at rest with the spring parallel to the inclined plane. The mass is set moving in simple harmonic motion. Does the period of oscillation depend on 𝜽?
7. A block of mass M slides a distance d from rest down a smooth inclined plane making an angle 𝜽 with the horizontal. It hits a light spring of natural length L and force constant k at the bottom of the incline. Determine the amount that the spring is compressed when the block comes to rest.
8. A bungee jumper of mass M is attached to an elastic cord of unstretched length L and force constant k. If they jump from rest how far do they fall before they come to rest?

Year 11 Physics Kinematics Sheet 1

A tutorial sheet of harder questions on kinematics is given below.

1. A car travels back and forth between two towns. The speed during the forward journey is v1. If the speed during the return journey is v2, what is the average speed for the entire trip?[2v₁v₂/(v₁+v₂)]
2. A car travels back and forth between two towns. The average speed during the forward journey is 50km/h. What is the speed for the return journey if the average speed for the entire trip was 40 km/h?
3. An object moves from rest with constant acceleration for 4.0 s then with uniform velocity for 5.0s, the total distance travelled being 224 m. What was the acceleration?
4. Two airports, A and B, are 850 km apart with B due east of A. At 10 am a jet takes off from A and travels at a constant velocity of 400 km/h to the east. At 11 am an aircraft takes off from B and travels at a constant velocity of 500 km/h to west. At what time do they pass each other?
5. Two camels leave the Sphinx at times 52.5 s apart. The first moves off with a uniform acceleration of 1.30 ms^-2 which it maintains for 1.92 s. It then continues with the acquired speed. The second camel moves off with a uniform acceleration of 1.45 ms^-2 which it maintains for 2.84 s. It then continues with the acquired speed. Calculate the time taken by the second camel to overtake the first camel and the distance travelled in this time.[82.9 s, 336 m]
6. A moving trolley is observed to have the following positions at equal intervals of 1 second; 5.10, 1.60, 1.50, 4.80, 11.5 and 21.6 cm. Show that the acceleration is uniform and determine its magnitude. What is the next number in the list? [3.40 cms^-2, 35.1 cm]
7. An elevator can increase its speed with an acceleration a and slow down with a deceleration b. What is the least time that it can travel a distance d if it starts and finishes at rest?
8. A stone is dropped down a well. The sound of the splash is heard after a time interval t. What is the depth of the well if the speed of sound is vs? Give the answer in terms of g, t and vs.

HSC Physics Transformers

Most HSC students can apply the transformer equation, but often the Physics principles are misunderstood. A tutorial sheet on transformers is given below.

1. What is the role of a transformer in the transmission of electricity?
2. Define one volt.
3. Define the term emf.
4. Is voltage is the same as emf?
5. What is the role of the iron core in a transformer?
6. What is meant by a soft iron core? Why is the soft iron core laminated?
7. Can a transformer operate using DC? Sketch a graph showing the primary and secondary voltage (on different axes) when the DC voltage applied to the primary coil is switched on and then after a short time switched off.
8. A common examination response is that "the induced emf in a coil equals the rate of change of the magnetic flux through the coil". Why is this incorrect?
9. When AC enters the primary coil of a transformer in which coil is an emf induced? Primary, secondary or both?
10. Assuming that the primary coil has low resistance, is the voltage applied to the primary coil equal to the counter emf induced in the primary coil? Is a counter emf induced in the secondary coil?
11. Explain how an emf is induced in the secondary coil.
12. The transformer equation uses the symbols Vp and Vs. Since transformers use AC, in which the current varies with time, what do these symbols represent?

HSC Physics Newton's Cannon

A tutorial sheet of harder problems on Newton's cannon is given below. Neglect air resistance, the rotation of the Earth and the movement of the Earth about the Sun.

1. Define escape speed.
2. Derive the equation for escape speed in terms of G, M and r.
3. Express escape speed in terms of g, G and r.
4. Does escape speed depend on the mass of the projectile m? Explain why in words.
5. Does escape speed depend on the launch angle?
6. Calculate the value of the escape speed from the surface of the Earth.
7. A cannon on a tall mountain fires a projectile horizontally at a small speed. Explain why the projectile falls to the Earth.
8. A projectile is fired from the surface of the Earth at 5 kms^-1 at 45° to the horizontal. Does this projectile go into orbit around the Earth?
9. The escape speed from the surface of a planet of radius R is V. A projectile is thrown vertically upwards at a speed V/2. Determine the maximum height reached by the projectile.
10. A tall mountain has an altitude h. The radius of the Earth is R and its mass is M. A cannon fires a shell horizontally at a speed u from the top of the mountain. Find the speed with which the shell strikes the Earth.
11. *A projectile is given an initial speed of 5 km/s at 45° to the horizontal from the surface of the Earth. Find the range on the surface of the Earth and the time of flight. What launch angle gives maximum range on the surface of the Earth? Take g=9.81 m/s² and R=6378 km [3121km, 1052s, 38°, 3219km 2Rsin^-1[v²/(2gR-v²)]]
12. *In the previous question the projectile is fired from a point on the equator towards the east. Taking into account the rotation of the Earth, how are the previous answers modified? [decreases (2706km), takes longer (1135s), lower angle (33.6°), decreases (2972km)]

HSC Physics Magnetic Force

When a current flows in a conductor that is placed in a magnetic field a magnetic force arises. Does the magnetic force act on the conductor or on the current?

IB Physics Electric Field

When a current flows through a wire is there an electric field inside the wire? Is there an electric field outside of the wire? When a spherical metal shell is given a static charge is there an electric field inside the shell? Is there a field in the metal? Is there an electric field outside of the shell?

HSC Physics Blackbody Radiation

The term black body radiation sometimes causes confusion in examination answers. Below is a tutorial sheet on this topic.

1. Define the term blackbody. Is the "blackbody" the object, a cavity in the object or a small hole in the wall of the object?
2. Define the term radiation as used in this context.
3. Is the Sun a perfect blackbody? Is a lump of hot coal a perfect blackbody? Is the filament of a light globe a perfect blackbody? Is a hot oven a perfect blackbody?
4. Sketch a graph showing the energy released per unit wavelength on the Y axis and wavelength on the X axis for a blackbody at a constant temperature. Is this graph the same shape as the "normal curve"? Is this graph the same shape as the distribution of particle speeds curve in an ideal gas?
5. Sketch a graph showing the number of oscillators (vibrating atoms in the walls of a hot object) versus the energy of each oscillator at a constant temperature. This is a key difference between the quantum theory of light and the wave theory of light. The classical wave theory of light assumes that every oscillator has the same kinetic energy at a given temperature and this (incorrect) assumption leads to the ultraviolet catastrophe.
6. Sketch a graph showing the energy released per unit frequency versus frequency for a blackbody at a constant temperature.
7. Sketch a graph showing the peak wavelength in the black body spectrum versus the temperature of the object.
8. What does the equation E=hf mean?
9. Quantum theory predicts that an oscillator can only have certain particular energies. These are given by E=(n+1/2)hf where n=0,1,2,3.... Why is this result different to E=hf?
10. *A perfect blackbody has a temperature of 5778 K. What fraction of the total energy released is contained between the wavelengths of 380 nm and 700 nm? This is the range of visible wavelengths coming from a blackbody at the same temperature as the photosphere of the Sun. [39%]
11. In the previous question the temperature is doubled. Is the percentage of radiation emitted as visible light increased, decreased or the same? [decreased, 31%]

IB Physics Relativity

Here is part of a question on Special Relativity taken from Columbia University's Physics Qualifying Examination in 2016. It was set by renown physicist and author Brian Greene. HL students aiming for a 7 must do extra reading to broaden their knowledge of Physics. Professor Greene's books make a good start. An asteroid is on a collision course with a space station located 5000 light minutes from the Earth. The asteroid is moving away from the Earth toward the space station at a speed 3/5c along a trajectory which is a straight line connecting the Earth and the space station. To save the station, NASA launches a missile from Earth at 4/5c. When the missile is launched, NASA determines that the asteroid is 400 light-minutes from Earth. How many minutes should NASA set on a timer located on the missile so that it will explode just as it catches the asteroid? [1200 minutes]

HSC Physics Assessment Task

A common Assessment Task in HSC Physics is to release a ball bearing from rest on a track inclined at a small angle to the horizontal and measure the time taken (t) to move various distances (d). The average speed is then calculated (v=d/t) and a graph of v-t is drawn. A common mistake is to then say that the gradient of this graph is the acceleration of the ball. Why is this incorrect? How do we determine the acceleration of the ball?

IB Physics Magnetic Field

When a current flows in a wire a magnetic field is created around the wire. Is there a magnetic field inside the wire?

IB Physics Electron Beams

Two straight parallel wires carrying currents in the same direction attract each other. Two electron beams travelling in the same direction repel each other. Why?

HSC Physics Binding Energy

One of the most misunderstood concepts in the quanta to quarks section of the current NSW HSC Physics syllabus is binding energy. Listed below are some tutorial points on this concept. 

1. The binding energy of a nucleus is the work that must be done on the separate nucleons to assemble them from an initial state where they do not influence each other into the final nucleus.
2. Binding energy has a negative sign.
3. A lone proton or neutron has no binding energy.
4. The total energy of a nucleus is the sum of the rest energy of each component particle and the binding energy of the nucleus.
5. Energy is released in a nuclear reaction when the product nuclei are more strongly bound (more stable) than the initial nuclei (less stable).

HSC Physics Band Theory

One of the most difficult and misunderstood concepts in the ideas to implementation section of the current NSW HSC Physics syllabus is band theory. Listed below are some tutorial points on this concept. 

1. Bands are not the same as the orbits of electrons around a nucleus (these are called shells).
2. Bands refer to an entire material not a particular atom.
3. The valence band is the set of possible energy states an electron can have if it is bonded to an atom. In a full valence band conduction is not possible as there are no states available for an electron to move to (no net flow of electrons).
4. The conduction band is the set of possible energy states that an electron can have if it is free of an atom. In a full conduction band conduction is not possible.
5. In a conductor at room temperature there is a partial overlap of the valence and conduction bands. The valence band is full and the conduction band is partially filled. Electrons can be moved easily by an applied electric field and enter nearby unoccupied states in the conduction band. In a semiconductor at room temperature there is a small energy gap between the valence and conduction bands. The valence band has some unoccupied states and some electrons occupy states in the conduction band. An electron in the valence band can gain thermal energy and enter the conduction band creating a hole in the valence band that behaves like a positive charge carrier. In an insulator there is a large energy difference between the valence and conduction bands. The valence band is full and the conduction band is empty.

HSC Physics Simultaneity

One of the most difficult and misunderstood concepts in the space section of the current NSW HSC Physics syllabus is simultaneity. Listed below are some tutorial points on this concept. Two sentences from Einstein's own popular exposition on relativity, Relativity, the Special and General Theory, Methuen, 1920, are given italics.

1. A frame of reference is a measurement system (coordinate system) where time and position values of an event can be determined. A railway platform is a frame of reference. A moving train is a frame of reference. A person on the platform makes measurements in the platform reference frame. A passenger on the train makes measurements in the reference frame of the train.
2. Two events are simultaneous in a reference frame if each has the same time coordinate in this reference frame.
3. Every reference body (coordinate system) has its own particular time; unless we are told the reference body to which the statement of time refers, there is no meaning in a statement of the time of an event.
4. Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity).
5. Suppose that two simultaneous events (such as lightning strikes) occur at the different points x1 and x2 in the reference frame of the platform. The times at which these events occur in the reference frame of the train are not the same as those in the reference frame of the platform. The time interval between these events in the platform reference frame is zero. The time interval between these events in the reference frame of the train is -v/c²(1-v²/c²)^-1/2(x2-x1). This expression shows that the time interval between two events depends on the location of the events and the relative speed of the reference frames. The factor c is the speed of light in a vacuum, this being the same in all inertial (non accelerating) frames of reference.

HSC Physics Back EMF

During the next 4 weeks I will list the most misunderstood concept in each of the four sections of the current NSW HSC Physics syllabus. Back emf is the most misunderstood concept in the motors and generators section. Here are some tutorial points on back emf.

1. Look at a demonstration motor and a generator. Draw diagrams of these labelling the parts.
2. When the magnetic flux through a coil changes an emf is induced in the coil.
3. Imagine a coil of wire (solenoid) carrying a constant current. As the current flows through the windings it makes a magnetic field that passes through the coil. If the current is turned off it does not drop to zero immediately. For a short time the current keeps flowing at a value that exponentially falls towards zero. During this time interval an emf is induced in the coil due to the changing magnetic flux through it causing the current to "keep going" for a short time, opposing the change that is happening. Sketch graphs showing (A) the current flowing in the coil versus time, (B) the potential difference across the coil versus time.
4. As the coil of a motor rotates in a magnetic field the changing magnetic flux through it induces an emf in the coil in accordance with Lenz's law. This emf tends to make an induced current in the opposite direction to the current supplied to the coil and is called a back emf.
5. When a motor is first turned on the current in the coils is large as there is no back emf. As the coils gain speed the back emf increases and the current in the coil decreases.

IB Physics Revision 2017

The IB Physics examination papers are on October 31 and November 1. From now until this date I will list questions to help students revise for their Physics examination. This list will be updated over the next six weeks.

Students can increase their marks significantly during this period if they are positive, organised and have a plan. What is required is a balanced approach to study with adequate nutrition, sleep, exercise and recreation.

1. Powers of Ten The radius of a proton is 0.88 fm. What is the density of a proton in kg/m³? (A) 10^-6, (B) 10², (C) 10¹⁰, (D) 10¹⁸, (E) 10²⁶
2. Uncertainty Analysis The measured uncertainty in the length of a pendulum is 2%, the uncertainty in its mass is 3% and the uncertaity in the acceleration due to gravity is 4%. What is the percentage uncertainty in its period of oscillation? (A) 1% (B) 2%, (C) 3%, (D) 4%, (E) 5%
3. Dimensions Surface tension is force per unit length. What are the dimensions of surface tension? (A)ML^-2 (B)MT^-2 (C)ML^-2T^-2 (D)L^-1T^-2 (E) ML^-1T^-2
4. Relative Velocity At noon a ship S is 10 km west of a tanker T. The velocity of S relative to the Earth is constant at 12 km/h N30°E and the constant velocity of T relative to the Earth is 4.0 km/h S60°W. What is the distance between the vessels at 3pm on the same day? (A)3.2 km (B) 21 km (C) 41 km (D) 48 km (E) 58 km
5. Change in Velocity A particle moves at a constant speed v around a circular path. What is the magnitude of its change in velocity after it turns through an angle 2𝜽? (A) vcos𝜽 (B) vsin𝜽 (c)2vcos𝜽 (D) 2vsin𝜽 (E)vsin2𝜽
6. Constant Acceleration A stone is thrown vertically upwards from the edge of a building and strikes the ground after a time t₁. When the stone is thrown vertically downwards from the same point at the same initial speed it hits the ground after a time t₂. The time to fall if the stone was released from rest is (A) t₁+t₂ (B) t₁-t₂ (C) (t₁t₂)^1/2 (D) (2t₁t₂)^1/2 (E) (0.5t₁t₂)^1/2
7. Resultant Force A marble dropped from a height h above soft sand penetrates a distance d into the sand before comng to rest. If the marble is dropped from a height 2h the distance that it travels in the sand before coming to rest is (A) d (B) 2d^1/2 (C) (2d)^1/2 (D) 2^1/2d (E) 2d
8. Stretching a Spring An unstretched spring of force constant k has one end tied to a vertical wall and a mass m at its other end. A constant horizontal stretching force F is applied to the mass. The extension of the spring in the horizontal direction when the mass comes to rest is (A) F/2k (B) F/k (C) 2F/k (D) 3F/k (E) 4F/k
9. Explosion A mass M is initially at rest. It breaks up into two equal pieces having a total kinetic energy E. What is the magnitude of the relative velocity of the two pieces after explosion? (A)(2E/m)^1/2 (B) (4E/m)^1/2 (C) (8E/m)^1/2 (D) (2m/E)^1/2 (E) (4m/E)^1/2
10. Ideal Gas Boltzmann's constant is equal to (A) R/N_A (B) RN_A (C) N_A/R (D) R+N_A (E) R/N_A²
11. Ideal Gas Particles A cylinder contains a volume V of helium gas at a temperature T. A second cylinder contains a volume 2V of argon gas at a temperature T. (A) the average speed of a helium atom is the same as that of an argon atom, (B) the average speed of a helium atom is less than that of an argon atom, (C) the average speed of a helium atom is greater than that of an argon atom, (D) each container contains the same number of particles, (E) the pressure exerted by each gas is the same
12. Change of State A copper block is heated in a Bunsen flame. The copper block is then placed on a large block of ice at 0°C in which it becomes half buried. What was the initial temperature of the copper block? (A) 5°C (B) 22°C (C) 42°C (D) 62°C (E) 82°C
13. Specific Heat Capacity One kilogram of copper and the same mass of water are heated using the same Bunsen burner. (A) the rate of temperature increase of both objects is the same, (B) the rate of temperature increase of water is greater than that of copper, (C) the rate of temperature increase of copper is greater than that of water, (D) the rate of internal energy increase is the same for each object, (E) the rate of internal energy increase for water is greater than that of copper.
14. Electric Current Two copper wires, P and Q, at the same temperature have the same battery of zero internal resistance connected across each of them. The length of P is twice that of Q. The drift speed of the electrons in P is (A) the same as that in Q, (B) greater than that in Q, (C) less than that in Q (D) is zero (E) depends on the diameter of the wire.
15. Resistance Four equal resistors R form the sides of a square. Another resistor R is placed across a diagonal of the square. What is the total resistance between the other two corners of the square? (A)R/4 (B) R/2 (C) R (D) 2R (E) 4R
16. Electric Field Strength Point charges of +4Q and -Q are placed a distance d apart in a vacuum. The resultant electric field is zero at a distance (A)2d to the left of -Q (B) d to the left of -Q (C) d to the right of -Q (D) 1.5d to the right of -Q (E) 2d to the right of -Q
17. Electrical Power A battery of emf 12V and constant internal resistance is connected to a 6Ω resistor. The power of the 6Ω resistor is greatest when the internal resistance is (A) 0Ω (B) 3Ω (C) 6Ω (D) 9Ω (E) 12Ω
18. Multiple Point Source Interference Monochromatic, coherent light is produced by two point sources. The intensity of the maxima produced on a screen is I. The experiment is repeated using 4 point sources with the same source spacing. The intensity of the principal maxima on the screen is (A) I, (B) 2I, (C) 4I, (D) 8I (E) 16I
19. Secondary Maxima The number of secondary maxima between the principal maxima in the interference pattern produced by 4 coherent, monochromatic point sources of light is (A) 0, (B) 1, (C) 2, (D) 3, (E) 4
20. Interference Pattern Compared to the interference pattern produced by 2 coherent, monochromatic point sources the interference pattern produced by 8 sources of the same spacing (A) has brighter principal maxima that are closer together, (B) has brighter principal maxima that are farther apart, (C) has brighter principal maxima that are the same distance apart, (D) has principal maxima of the same intensity that are the same distance apart, (E) has principal maxima of the same intensity that are the closer together.
21. Single Rectangular Slit Diffraction Monochromatic light passes through a single narrow slit. The intensity pattern is observed on a screen at a large distance from the single slit. Why does the intensity not stay at zero after the central maximum?

HSC Physics Revision 2017

The HSC Physics examination is on October 30. From now until this date I will list questions to help students revise for their Physics examination. This list will be updated during the next two months.

Students can increase their marks significantly during this period if they are positive, organised and have a plan. Remember that the HSC Physics examination is not "difficult Physics". What is required is a steady approach to study with adequate nutrition, sleep, exercise and recreation. 

1. Accelerating Reference Frame A train moving in a straight line on horizontal ground is accelerating to the east. A ball is released from rest (relative to the floor of the train) from a height of 2.0 m. Sketch the path of the ball in the reference frame of the (i) train, if the train is moving to the east (ii) Earth, if the train is moving to the east (iii) train, if the train is moving to the west, and (iv) Earth, if the train is moving to the west.
2. AC Motor An AC generator is reversed so that AC is now fed into the coil through the brushes that rub against the slip rings. Does the coil spin?
3. Photocells and Solar Cells Describe the differences between a photocell and a solar cell.
4. Cloud Chamber Describe how a cloud chamber detects the presence of radiation. Which radiation produces the (i) thickest tracks? (ii) thinnest tracks? (iii) straightest tracks? (iv) Can gamma rays be directly observed in a cloud chamber?
5. Earth Satellite A satellite is in orbit above the Earth's equator. Determine the altitude of the satellite if it (i) appears stationary above the equator, (ii) appears to pass overhead (as seen by a person on the equator) from west to east every 8 hours, (iii) appears to pass overhead from east to west every 8 hours.
6. Magnetic Field of Two Currents Two long straight parallel wires at a distance d apart carry currents of I and 4I in opposite directions. Where is the resultant magnetic field strength zero?
7. Heinrich Hertz Describe how Hertz discovered the photoelectric effect.
8. Strong Nuclear Force Sketch graphs showing the force between two nucleons and the potential energy of two nucleons as a function of their distance apart.
9. Artificial Gravity A space station far away from any planet or star has the shape of a torus of outer radius 500 m. It rotates about an axis through its centre and perpendicular to its plane at constant rate of 10 rev/min. An astronaut of mass 80 kg is standing on the inside of the outer surface of the space station (i) describe the force/s acting on the astronaut, (ii) determine the weight of the astronaut (iii) an apple is released from rest inside the spacestation at a distance of 1 m from the outer wall. Describe the subsequent motion of the apple.
10. EMF and Back EMF What is the difference between an induced EMF (as in a generator) and a back EMF (as in a motor)?
11. CRO Describe how an electron beam produces a two dimensional image on the screen of a cathode ray oscilloscope.
12. Electron in the Hydrogen Atom According to quantum physics, how do we describe the electron in the hydrogen atom?
13. Maximum Range A basketball is thrown at a speed U an angle 𝜽 to the horizontal from of a height h above the ground. Show that the angle that gives the maximum range on the ground is given by csc²𝜽_max = 2(1+gh/U²).
14. Magnetic Force Two electrons are moving at the same velocity v relative to the laboratory side by side in parallel paths a distance d apart in the laboratory reference frame. Determine the magnitude of the resultant force between the electrons in the (a) reference frame of the electrons, (b) laboratory reference frame.
15. Planck and Einstein Outline the political views of Max Planck and Albert Einstein. How did this affect their scientific work?
16. Enrico Fermi Assess the contribution of Enrico Fermi to Physics.
17. High Speed Electron An electron is accelerated from rest through a potential difference of 100,000 V using a particle accelerator. Find the final speed of the electron.
18. Loudspeaker Describe how a loudspeaker uses the motor effect to produce sound waves.
19. Blackbody Radiation Curve Explain why the blackbody radiation curve has a peak at a certain wavelength. Does a photon of this wavelength have the greatest energy?
20. Linear Accelerator Describe how a linear accelerator produces a high speed particle.
21. Projectile Practical A marble is projected horizontally at a known constant speed from a spring gun at various heights above the floor and the horizontal range is measured in each case. (i) Plot this data so that a straight line graph is produced, (ii) how can the graph be used to determine g?
22. Step-Up Transformer A step-up transformer is used to increase the voltage of an AC supply with respect to the ground. Explain how this reduces reduces the heat loss along the transmission line from the power station.
23. Phonons A student in an answer to a question on the BCS theory mentions a "phonon". Outline what a phonon is.
24. Forces of Nature Outline the force that holds together, giving the carrier particle in each case (i) the nucleons in an alpha particle, (ii) the quark combination that produces a proton
25. Michelson and Morley Describe the result of the Michelson and Morley experiment.
26. Torque and Work Describe the difference between torque and the work done by a force.
27. n-type Semiconductor Is an n-type semiconductor negatively charged?
28. Binding Energy The mass of the two protons, two neutrons and two electrons is greater than their combined mass when they form a helium atom. Describe why this is so.
29. Gravitational Potential Energy When is the gravitational potential energy of two masses least? When they are close together or far apart? Explain.
30. Cathode Rays Describe Thomson's experiment to measure the charge to mass ratio of cathode rays.
31. Bragg Diffraction Using a labelled diagram, explain using wave concepts how constructive interference occurs when x-rays strike a solid object.
32. Line Spectrum Explain how the presence of a line spectrum indicates the existence of energy levels in the hydrogen atom.
33. Orbital Decay In an answer a student states that the speed of an orbiting satelite decreases when it is subjected to air drag. Is this correct?
34. Back EMF Does the back emf eventually stop the coils of a DC motor from spinning? Explain.
35. Electromagnetic Waves Describe how Heinrich Hertz experimentally produced radio waves and identified these as belonging to the same group as light waves.
36. Moderator A neutron given off in a fission reaction has a kinetic energy of 6.0x10^-13 J. This is reduced to 6.0x10^-21 J by causing the neutron to make a series of collisions with carbon nuclei in the moderator. The fractional loss of kinetic energy of a neutron at each collision is 0.14. Find the number of collisions involved in this process. [122]
37. Polar Orbit A satellite is placed in an orbit that passes over points near the north and south poles of the Earth. Does this satellite possess more energy than a similar satellite in orbit at the same altitude above the equator?
38. Power Which combination has the higher power? A large resistance R connected to a battery or a smaller resistance r?
39. Carbon Carbon has 4 electrons in its outer shell. (a) is the outer shell the same as the valence band? (b) is its valence band full? (b) is carbon a semiconductor?
40. Electron Wavelength Compared to an electron in the ground state, the wavelength of an electron in the first excited state of the hydrogen atom is (A) 1/4, (B) 1/2, (c) the same, (D) twice as large (E) 4 times as large
41. Acceleration A student doing a physics problem calculates the acceleration of an electron as 2.0x10¹⁴ ms^-2. Is this possible?
42. DC Motor A DC motor has a split-ring commutator and the coils are in a radial magnetic field. An open switch is in the circuit. The switch is now closed. Sketch graphs showing (A) the current flowing in the coils in terms of time (B) the torque exerted by the magnetic field on the coil in terms of time.
43. Band Structure A conductor and a semiconductor are at room temperature. Choose the correct statement.(A) the conduction band of the conductor is empty (B) the conduction band of the semiconductor is empty (C) the conduction band of a semiconductor is partially filled (D) the conduction band of a conductor is full (E) the valence band of a semiconductor is full
44. Neutron A neutron (A) is not affected by a magnetic field (B) has no spin about its axis (C) has a magnetic moment causing it to align itself with a magnetic field (D) has zero magnetic moment since it has zero charge (E) is made of a proton and an electron.
45. Orbit A satellite of total mass m moves at a constant speed v in a circular orbit around a planet. It suddenly fires a projectile of mass 0.5m at a speed of 0.5v in the opposite direction to its initial velocity. After it launches the projectile the satellite (A) continues in the same orbit with the same speed, (B) moves to a higher orbit (C) escapes from the planet, (D) moves to a lower orbit, (E) continues in the same orbit with a shorter period
46. Soft Iron Core The role of the iron core in a transformer is to (A) reduce the eddy currents, (B) connect together the primary and secondary coils, (C) increase the magnetic flux passing through the coils, (D) carry all of the magnetic field of the primary coil through the secondary coil, (E) change the voltage of the primary coil.
47. Type 1 and 2 Superconductors Superconducting magnets use (A) type 1 superconductors since B=0 inside them, (B) type 1 superconductors since B≠0 inside them, (C) type 2 superconductors since B≠0 up to a certain value, (D) type 2 superconductors since B=0 inside them, (E) both type 1 and type 2 superconductors.
48. Bohr's Postulate In 1913 Niels Bohr proposed that mvr = n h/(2𝜋), where n is the principal quantum number, m is the mass of the electron, v is the speed of the electron and r is the radius of the stationary state. (A) the original Bohr theory is still used to explain the hydrogen atom, (B) Bohr's equation gives the magnitude of the electron angular momentum that agrees accurately with modern experiments, (C) Bohr's equation was replaced since quantum mechanics predicts that for each value of n there are n possible values of the orbital angular momentum, (D) the electron is now considered to have non-zero orbital angular momentum in its ground state, (E) the electron is now considered to have zero orbital angular momentum for every value of n.
49. Projectile1 A ball is thrown at 49m/s at 60° to the horizontal. Neglecting air resistance, the times after projection when the velocity vector makes an angle of 30° with the horizontal are (A)2.16s, 6.50s (B) 2.33s, 6.33s (C) 2.89s, 5.77s (D) 3.66s, 5s (E) 4.00s, 4.66s.
50. Projectile2 A ball is thrown at 49m/s at 60° to the horizontal. Neglecting air resistance, the times after projection when the speed of the ball is 35m/s are (A) 0.48s, 8.18s (B) 1.10s, 7.56s (C) 1.78s, 6.88s (D) 2.56s, 6.10s (E) 4.00s, 4.66s.
51. Air Resistance The drag force acting due to air resistance on a projectile is proportional to the square of the speed of the projectile. A projectile is thrown vertically upwards at 49m/s. The maximum height reached is 60m. The projectile is now thrown at the same speed at an angle to the vertical. The maximum possible range of the projectile on the horizontal is (A) 30m (B) 60m (C) 94m (D) 120m (E) 180m
52. Speed A projectile is thrown at 49m/s at 60° to the horizontal in the absence of air resistance. The gradient of the distance travelled-time graph for the projectile has a maximum value of (A) 98m/s, (B) 75m/s, (C) 49m/s, (D) 42m/s, (E) 24.5m/s.
53. Average Speed Two identical projectiles, P and Q, are thrown on level ground at the same speed. P is thrown at 30° to the horizontal and Q is thrown at 60° to the horizontal. Neglecting air resistance, (A) the range of P is greater than Q, (B) the average speed of each projectile is the same during their flight, (C) the average speed of P is greater than the average speed of Q, (D) the average speed of P is less than the average speed of Q, (E) the average velocity of each object is the same during the flight.