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?

1 What force would be needed to produce an acceleration of 4m/s² in a ball of mass 6kg? 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? 4 When can we say that one body interacts with the second body? 5 Define the force of friction.