Work equals the change in kinetic energy.
Change in kinetic energy = Final kinetic energy - Initial kinetic energy

If a body is initially at rest, then work done will equal the kinetic energy of the body

When the area below a force versus displacement graph is greater than the area above it, the total work done will be negative and the body will have lost kinetic energy.

Kinetic Energy can be calculated using

where

E_{k} = kinetic energy (J),

m = mass, (kg)

v = velocity or speed, (m/s)

Sample Problem
How much work must be done to accelerate a 1200 kg truck from 13 m/s to 28 m/s?

Solution:
Since the truck is not initially at rest we must subtract the initial kinetic energy from the final kinetic energy of the truck in order to calculate work done.

An elastic collision is one in which there is no change in kinetic energy after the collision has occurred. In other words, kinetic energy is conserved.

Ideal gas molecules

The following examples are of collisions that are very nearly elastic in nature

Billiard balls

Baseball and bat

An inelastic collision is one where some energy is "lost" when colliding objects are in contact. This "lost" or dissipated energy usually results in the production of heat. It may also produce sound, light, or other forms of energy.

In some cases the shape of the colliding objects may be permanently changed.

Completely inelastic collisions occur when two bodies stick together.