Energy transformations in the universe over time are (generally) characterized by various kinds of energy which has been available since the Big Bang, later being "released" (that is, transformed to more active types of energy such as kinetic or radiant energy), when a triggering mechanism is available to do it.

Release of energy from gravitational potential: A direct transformation of energy occurs when hydrogen produced in the big bang collects into structures such as planets, in a process during which part of the gravitational potential is to be converted directly into heat. In Jupiter, Saturn, and Neptune, for example, such heat from continued collapse of the planets' large gas atmospheres continues to drive most of the planets' weather systems, with atmospheric bands, winds, and powerful storms which are only partly powered by sunlight, however, on Uranus, little of this process occurs.

On Earth a significant portion of heat output from interior of the planet, estimated at a third to half of the total, is caused by slow collapse of planetary materials to a smaller size, with output of gravitationally driven heat.

Release of energy from radioactive potential: Familiar examples of other such processes transforming energy from the Big Bang include nuclear decay, in which energy is released which was originally "stored" in heavy isotopes, such as uranium and thorium. This energy was stored at the time of these elements' nucleosynthesis, a process which ultimately uses the gravitational potential energy released from the gravitational collapse of type IIa supernovae, to store energy in the creation of these heavy elements before they were incorporated into the solar system and the Earth. 

Mechanical energy describes the ability of an object to do work. The work done on an object arises from a force applied over a distance (W=Fdd) which either accelerates the object thus changing its motional energy (kinetic energy), or stores energy by changing its position (potential energy). For instance, when a moving car is brought to rest, the work done by the frictional force on the tires is equal to the kinetic energy of the car, KE=1/2 mv2. In addition, forces which are provided by the car's engine can do work in climbing up a hill which is stored as gravitational potential energy, PE=mgh. The mechanical energy of an object is equal to the sum of the potential plus kinetic energies, i.e. E = PE + KE, and is a direct measure of the total energy available to an object as its speed and position changes from one point to another.

In special cases where energy is not lost to the environment due to nonconservative forces such as friction, the mechanical energy of a system of masses remains constant as the object or objects move in space. In other words, the energy of the system stays the same for all times in the life of the objects. Since there is no energy created or destroyed as the objects travel along on their paths, the forces which do work exchange energy between potential and kinetic according to the energy initially provided to the system. This is the basis for the law of  conservation of energy which can be written as E = PE + KE = constant. As an example, consider an apple of mass m which is initially a height h above the ground. When the apple is hanging from the tree, it stores an amount of gravitational potential energy as it is held motionless so that the initial mechanical energy is Etop = PEtop = mgh. If the apple is cut from the tree, forces due to gravity do work which cause it to fall and speed up at the same time. Just before it hits the ground, the potential energy is zero such that the mechanical energy arises solely due to the kinetic energy, Ebot = KEbot = 1/2 mv2. Using the conservation of energy for this frictionless problem, we can equate the mechanical energy at the top and bottom of the tree, or more specifically                                       E top =E top

PE top = KE bot

mgh = 1/2 mv2.

It is clear from the equations above that the mechanical energy allows you to relate the position of the apple to its velocity as it falls from the tree. As potential energy is lost from the decrease in height above the ground, kinetic energy is gained while the object speeds up.

 

EX. #1: A helicopter drops a 20 kg package from rest at a height of 120 m from the ground.

a) How much initial potential energy is stored by the package?

b) What is the kinetic energy of the package just before it hits the ground?

c) What is the speed of the package just before it hits the ground?

SOLUTION:

a)      The distance of the package from the ground is given as 120m. This provides a reference potential energy at the ground of zero since h=0 when the object hits the ground below the helicopter. If the zero of gravitational potential energy is defined this way, then the potential energy of the package can be written as

PEgrav=mgh = (20 kg)(10 m/s2)(120 m) = 24000 J

b)      By ignoring any energy lost due to friction, we can use the conservation of energy to solve for the energy after the package hits the ground. Since the package is held motionless above the ground, we see that the initial mechanical energy is simply the potential energy it has while in the helicopter (no KE):

Etop = PEtop + KEtop = 24000 J

As the object falls and hits the ground at the bottom, it loses all of the potential energy (reaches the point where h=0) which is transformed into kinetic:

Ebot = PEbot + KEbot = KEbot

Now applying the concept of energy conservation, we equate the energy at the top to the energy at the bottom to get the final answer

Ebot = Etop

KEbot = 24000 J

Notice that this simply says that all of the stored potential energy the package had while in the helicopter is transformed into kinetic just before it hits the ground.

c)      Since we know the definition of the kinetic energy KE=1/2 mv2, we just need to solve for the speed of the package by setting the answer above equal to the KE formula:

KE=1/2 mv2 =1/2 (20 kg) v2 = 24000 J

v2= 2400 m2/s2                        v = 48.99 m/s

 

There are many different machines and transducers that convert one energy form into another. A short list of examples follows:

Thermoelectric (Heat → Electric energy)
Geothermal power (Heat→ Electric energy)
Heat engines, such as the internal combustion engine used in cars, or the steam engine (Heat → Mechanical energy)
Ocean thermal power (Heat → Electric energy)
Hydroelectric dams (Gravitational potential energy → Electric energy)

Energy neither be created nor be destroyed it can be converted from one form of energy into other form of energy.....this conclusion we can give for this..