The mass of a body is a measure of the resistance of the body to a change in its velocity. A force is an action which causes a body to accelerate.
Friction is a contact force that opposes the relative motion of two surfaces.
The kinetic energy is a measure of the amount of work needed to increase the speed of the body from zero to v. The work done to lift a body of weight m.g through a height h is m.g.h. The sum of the kinetic energy and the potential energy remains constant. This is called the conservation of energy . The general principle of conservation of energy states that energy cannot be created or destroyed .