The capacity of doing work that is acquired by a moving body due to its motion is called kinetic energy.
In everyday language, energy means a capacity for activities and hard work. Likewise, in physics, energy is a capacity for performing work. Energy is “stored” work, which can be converted into actual work under suitable conditions. For instance, a body in motion, such as a speeding arrow, has the energy of motion, or kinetic energy. This energy will be converted into work when the arrow strikes a target. A highspeed arrow has deeper penetration and delivers a larger amount of work to the target. Thus, we see that the kinetic energy of the arrow, or the kinetic energy of any kind of particle, must be large if the speed is large.
The quantity 1/2mv2 is the amount of work stored in the body or the kinetic energy of the body. We represent the kinetic energy by the symbol K:
When a force does positive work on a body initially at rest, the kinetic energy of the body increases. The body then has a capacity to do work if the moving body subsequently is allowed to push against some obstacle, then this obstacle does negative work on the body and simultaneously the body does positive work on the obstacle. When the body does work, its kinetic energy decreases. The total amount of work the body can deliver to the obstacle is equal to its kinetic energy. Thus, the kinetic energy represents the capacity of a body to do work by virtue of its speed.