**Kinetic energy** From Wikipedia, the free encyclopedia

The

**kinetic energy** of an object is the

energy which it possesses due to its

motion.

^{[1]} It is defined as the

work needed to accelerate a body of a given mass from rest to its stated

velocity. Having gained this energy during its

acceleration,

the body maintains this kinetic energy unless its speed changes. The

same amount of work is done by the body in decelerating from its current

speed to a state of rest.

The speed, and thus the kinetic energy of a single object is

frame-dependent (relative): it can take any non-negative value, by

choosing a suitable

inertial frame of reference.

For example, a bullet passing an observer has kinetic energy in the

reference frame of this observer, but the same bullet is stationary, and

so has zero kinetic energy, from the point of view of an observer

moving with the same velocity as the bullet.

^{[2]} By contrast, the total kinetic energy of a system of objects cannot be

reduced to zero by a suitable choice of the inertial reference frame,

unless all the objects have the same velocity. In any other case the

total kinetic energy has a non-zero minimum, as no inertial reference

frame can be chosen in which all the objects are stationary. This

minimum kinetic energy contributes to the system's

invariant mass, which is independent of the reference frame.

The cars of a

roller coaster reach their maximum kinetic energy when at the bottom of their path.

When they start rising, the kinetic energy begins to be converted to

gravitational

potential energy. The sum of kinetic and potential energy in the system remains constant, ignoring losses to

friction.