Everything about The Kinetic Energy totally explained
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.
Etymology
The adjective "kinetic" to the noun
energy has its roots in the
Greek word for "motion" (
kinesis). The terms
kinetic energy and
work and 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.
Introduction
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's transformed from other forms of energy and to the other forms. 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 the energy of motion, known as
kinetic energy but the process isn't completely efficient and heat is also produced within the cyclist.
The kinetic energy in the moving
bicycle and the
cyclist 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'll never regain all of its speed without further pedaling. Note that the energy isn't 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 heat energy.
Like any physical quantity which is a function of velocity, the kinetic energy of an object doesn't depend only on the inner nature of that object. It also depends on the relationship between that object and the observer (in physics an observer is formally defined by a particular class of coordinate system called an
inertial reference frame). Physical quantities like this are said to be not
invariant. The kinetic energy is co-located with the object and contributes to its gravitational field.
Calculations
There are several different equations that may be used to calculate the kinetic energy of an object. In many cases they give almost the same answer to well within measurable accuracy. Where they differ, the choice of which to use is determined by the velocity of the body or its size. Thus, if the object is moving at a velocity much smaller than the speed of light, the
Newtonian (classical) mechanics will be sufficiently accurate; but if the speed is comparable to the speed of light,
relativity starts to make significant differences to the result and should be used. If the size of the object is sub-atomic, the
quantum mechanical equation is most appropriate.
Newtonian kinetic energy
Kinetic energy of rigid bodies
In
classical mechanics, the kinetic energy of a "point object" (a body so small that its size can be ignored), or a non rotating
rigid body, is given by the equation
where
is known as the Von Weizsacker kinetic energy functional.
Some 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's 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'll 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 kinetic energy is preserved.
Flywheels are being developed as a method of
energy storage (see article
flywheel energy storage). This illustrates that kinetic energy can also be rotational. Note the formula in the articles on flywheels for calculating rotational kinetic energy is different, though analogous.
Further Information
Get more info on 'Kinetic Energy'.
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