Define the force of friction.

3 When force applied by a moving vehicle is calculated, the actual force applied by the brakes may be slightly less than the one calculated theoretically. Why? 4 When can we say that one body interacts with the second body? 6 How do ball- bearings affect the nature of friction? Illustrate with an example. 7 When does the maximum loss in weight of an object immersed in water take place? Illustrate.