Energy possessed by an object by virtue of its relative position or state (for example, as in a compressed spring or a muscle). It can be thought of as ‘stored’ energy. An object that has been raised has stored energy due to its height. It is described as having gravitational potential energy.
If a ball is raised to a certain height and released, the ball falls to the ground. The potential energy changes to kinetic energy. As the ball hits the ground some of the kinetic energy is lost as sound and elastic energy. A stretched spring has stored elastic energy; this is known as elastic potential energy. Springs are designed to store energy and release it either rapidly or slowly. For example, a mechanical toy works by an unwinding spring coil inside the toy. As the coil unwinds, elastic potential energy changes into kinetic and sound energy as the toy operates.
A moving body has kinetic energy. This energy is equal to the work that would have to be done in bringing the body to rest, and is dependent on both the body's mass and speed. The kinetic energy in joules of a mass m kilograms travelling with speed v metres per second is given by the formula:
KE = 1/2mv2
If a moving object collides with another object, then work is done. For example, if a moving car collides with a stationary car, it will cause the stationary car to move. The force from the moving object is used to move the stationary object by a certain distance.
All atoms and molecules possess some amount of kinetic energy because they are all in some state of motion (see kinetic theory). Adding heat energy to a substance increases the mean kinetic energy and hence the mean speed of its constituent molecules – a change that is reflected in a rise in the temperature of that substance.
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