Energy is the ability of an object to cause change. The energy of an object in motion is called kinetic energy. The word "kinetic" was derived from the Greek word for motion. The greater the mass of a moving object, the greater is its kinetic energy. A 4,000 pound Cadillac moving at 60 miles per hour has more energy that a 2,000 pound Subaru. Kinetic energy is also related to speed. The greater the speed, the greater the kinetic energy. For example, a 2,000 pound Subaru moving at 120 miles per hour has more kinetic energy that a 4,000 pound Cadillac moving at 5 miles per hour.
Potential energy is the ability of an object to cause change because of its position. When two things are pulling on each other, the further apart you separate them, the greater the potential energy. For example, when a string on a bow is pulled away from the bow, you create potential energy which can be used to propel an arrow. If a rock is separated from the gravitational pull of earth by carrying it to the top floor of a building, it now has a great deal of potential energy. If you drop the rock from the building, it has the potential to change things. The potential energy an object has because of its position above the ground is called gravitational potential energy. Also, the greater the mass of the separated objects, the greater the potential energy. A bowling ball separated from earth by 50 meters can do more changing that a ping pong ball separated from earth by 50 meters.
If you separate an object from earth, it has potential energy because of this separation. If you drop the object, it gains kinetic energy as it loses potential energy. After dropping the object, potential energy is converted to kinetic energy. If you throw a ball upward, kinetic energy is converted to potential energy as the ball goes upward. When it reaches maximum height, it stops and loses all kinetic energy to potential energy. As the object begins to fall, its potential energy is converted to kinetic energy. This describes energy conversion. Energy conversion is the change of energy from one form to another. A pendulum swinging is an example of a rapid series of energy conversion between kinetic energy and potential energy, and potential energy and kinetic energy.
Kinetic energy and potential energy are types of mechanical energy. Mechanical energy is energy due to motion or position. We all know that a pendulum cannot swing forever. On earth, there is no such thing as perpetual motion. Most kinetic energy will eventually be converted to friction, and this friction will stop the object from moving. The less friction, the longer a moving object will continue to move. This is why lubricants such as oil and grease are used to lessen friction in moving parts.
Friction involves kinetic energy being converted to heat. If you rub two things together, the friction produces heat. Two pieces of ice rubbed together will cause enough friction to melt the ice, even if this is done where the temperature of the surrounding air is below zero.
In 1905 Albert Einstein published his theory of special relativity. It
dealt with the energy and mass of objects at very high speeds. This theory
states that no object can travel faster than the speed of light. It also states
that the mass of an object increases as its speed increases. E = MC2 is his
famous equation to relate energy to mass as the object approaches the speed of
light. E=energy, M=mass, and C=the speed of light. His theory states that mass
and energy are equivalent. When something gains energy, it gains mass. When
something gains mass, it gains energy. However, for everyday moving objects,
the change in mass as related to speed is undetectable.