# 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?