Friction.....Physics in the Real World

In many textbook problems we see the phrase "neglect air resistance" or "assume that all surfaces are smooth" in  projectile problems or mechanics problems. These assumptions allow problems to be "solvable"  using the well known equations of uniformly accelerated motion. Now let us step into the real world where dissipative agents that remove energy from a system act. What causes the frictional force between two surfaces? How can we explain friction at an atomic level? And, why is it "harder" to push start a heavy object than to keep pushing it along?  Finally, here is a discussion question on air friction. Imagine that a ball is thrown vertically upwards and the air resistance has the same size during the journey. Which stage of the flight, upward or downward, takes the longer time?

Single Slit Diffraction

Light plays a key role in Physics. The study of the interaction between light and matter has caused many breakthroughs in in Physics. At a fundamental level, waves spread out after they pass through a gap. This process is called diffraction. When laser light passes through a narrow rectangular slit, and the pattern observed on a screen, most of the energy is found arriving on the screen in a bright area called the central maximum. This is flanked on each side by a dark area and then the pattern becomes brighter on each side at a location called the first order maximum. The first order maximum has an intensity of 4.7% that of the central maximum. Higher order maxima also form, the next two of which have intensity ratios of 1.7% and 0.83% respectively. Why doesn't all of the energy "land" in the central maximum? Why do we get higher order maxima?