Energy Conservation

The change in the kinetic energy of an object is equal to the net work done on the object

Work Energy Principle

In order to accomplish work on an object there must be a force exerted on the object and it must move in the direction of the force.

This fact is referred to as the Work-Energy Principle and is often a very useful tool in mechanics problem solving. It is derivable from conservation of energy and the application of the relationships for and energy, so it is not independent of the conservation laws. It is in fact a specific application of conservation of energy. However, there are so many mechanical problems which are solved efficiently by applying this principle that it merits separate attention as a working principle. For a straight-line collision, the net work done is equal to the average force of impact times the distance traveled during the impact.

Average impact force x distance traveled = change in kinetic energy

If a moving object is stopped by a collision, extending the stopping distance will reduce the average impact force.

Example of Force on Car

Source : http://hyperphysics.phy-astr.gsu.edu