Kinetic Energy

The kinetic energy of an object is the extra energy which it possesses due to its motion. 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 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. 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 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 energy (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

History and etymology
The adjective "kinetic" has its roots in the Greek word κίνηση (kinesis) meaning "motion" – the same root as in the word cinema (referring to motion pictures).

The principle in classical mechanics that E ∝ mv² was first theorized by Gottfried Leibniz and Johann Bernoulli, who described kinetic energy as the "living force", vis viva. Willem 's Gravesande of the Netherlands provided experimental evidence of this relationship. By dropping weights from different heights into a block of clay, 's Gravesande determined that their penetration depth was proportional to the square of their impact speed. Émilie du Châtelet recognized the implications of the experiment and published an explanation.[1]

The terms "kinetic energy" and "work" with their present scientific meanings date back to the mid 19th century. Early understandings of these ideas can be attributed to Gaspard-Gustave Coriolis who in 1829 published the paper titled Du Calcul de l'Effet des Machines outlining the mathematics of kinetic energy. William Thomson, later Lord Kelvin, is given the credit for coining the term "kinetic energy" c. 1849 - 1851
There are various forms of energy: chemical energy, heat, electromagnetic radiation, potential energy (gravitational, electric, elastic, etc.), nuclear energy, rest energy. These can be categorized in two main classes: potential energy and kinetic energy.

Kinetic energy can be best understood by examples that demonstrate how it is transformed to and from other forms of energy. For example, a cyclist will use chemical energy that was provided by food to accelerate a bicycle to a chosen speed. This speed can be maintained without further work, except to overcome air-resistance and friction. The energy has been converted into kinetic energy – the energy of motion – but the process is not completely efficient and heat is also produced within the cyclist.

The kinetic energy in the moving cyclist and the bicycle can be converted to other forms. For example, the cyclist could encounter a hill just high enough to coast up, so that the bicycle comes to a complete halt at the top. The kinetic energy has now largely been converted to gravitational potential energy that can be released by freewheeling down the other side of the hill. (Since the bicycle lost some of its energy to friction, it will never regain all of its speed without further pedaling. Note that the energy is not destroyed; it has only been converted to another form by friction.) Alternatively the cyclist could connect a dynamo to one of the wheels and also generate some electrical energy on the descent. The bicycle would be traveling more slowly at the bottom of the hill because some of the energy has been diverted into making electrical power. Another possibility would be for the cyclist to apply the brakes, in which case the kinetic energy would be dissipated through friction as thermal energy.

Like any physical quantity which is a function of velocity, the kinetic energy of an object depends on the relationship between the object and the observer's frame of reference. Thus, the kinetic energy of an object is not invariant.

Examples
Spacecraft use chemical energy to take off and gain considerable kinetic energy to reach orbital velocity. This kinetic energy gained during launch will remain constant while in orbit because there is almost no friction. However it becomes apparent at re-entry when the kinetic energy is converted to heat.

Kinetic energy can be passed from one object to another. In the game of billiards, the player gives kinetic energy to the cue ball by striking it with the cue stick. If the cue ball collides with another ball, it will slow down dramatically and the ball it collided with will accelerate to a speed as the kinetic energy is passed on to it. Collisions in billiards are effectively elastic collisions, where (by definition) kinetic energy is preserved. In inelastic collisions, kinetic energy is dissipated as: heat, sound, binding energy (breaking bound structures), or other kinds of energy.

Flywheels are being developed as a method of energy storage (see Flywheel energy storage). This illustrates that kinetic energy can also be rotational.

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