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 current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. Negative work of the same magnitude would be required to return the body to a state of rest from that velocity.The kinetic energy of a single object is completely frame-dependent (relative): it can take any non-negative value, by choosing a suitable inertial 

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frame of reference. For example, a bullet racing by a non-moving observer has kinetic energy in the reference frame of this observer, but the same bullet has zero kinetic energy in the reference frame which moves with the bullet.

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By contrast, the total kinetic energy of a system of objects is not completely removable by a suitable choice of the inertial reference frame, unless all the objects have the same velocity. In any

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other case the total kinetic energy is at least equal to a non-zero minimum which is independent of the inertial reference system. This kinetic energ

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y (if present) contributes to the system's invariant mass, which is seen as the same value in all reference frames, and by all observers.

The kinetic energy of an object of mass m traveling at a speed v is mv2/2, provided v is much less than the speed of light.