When can we say that one body interacts with the second body?

2 A force acts on an object which is free to move. If we know the magnitude of the force and the mass of the object, which aspect of motion of the object does Newton's second law of motion enable us to determine? 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? 5 Define the force of friction. 6 How do ball- bearings affect the nature of friction? Illustrate with an example.