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The energy of an object depends on many factors. Different types and forms of energy are affected by different factors. For instance, kinetic energy depends on the mass and speed of an object. Gravitational potential energy depends on the mass of an object, but also on its height above the ground.

The kinetic energy of an object depends on both the mass and the speed of the object.

Kinetic energy is the energy due to motion. An object that is moving more quickly will have more kinetic energy than an identical object moving more slowly. For instance, imagine that you ask your friend to lend you a pen. If your friend gently tosses the pen to you, it will have much less energy than if your friend throws the pen with a lot of force.

The faster the pen is moving, the more kinetic energy it will have.

Increasing an object's mass will also increase its kinetic energy. Imagine a table tennis ball and a golf ball. The two balls are about the same size. However, if they were thrown at the same speed, the golf ball would be able to do more work because it has more mass.

The gravitational potential energy of an object depends on the object's height. The farther an object can fall, the more gravitational potential energy it has. The gravitational potential energy of an object also depends on the object's mass. If two objects are moving at the same speed, the more massive object will be able to do more work.

Because of this, even when objects are stationary, a massive object will have more potential energy than a less massive object at the same height. The gravitational potential energy of an object also depends on the strength of gravity. However, gravity is constant on the surface of Earth, so this is not a factor most of the time.

Like matter, energy is conserved. It is not created or destroyed. For this reason, we can track how energy moves through a system. Roller coaster cars can be treated as a system. The cars are lifted up a hill, which gives the system an input of energy. This energy then transforms from potential energy to kinetic energy and back again as the cars go down and up hills on the track. Because no more energy is added to the system during the ride, we know that the cars will never be able to go higher than the peak of the first hill. Not only can the cars go no higher, but some energy is lost due to friction, so the height of hill that the cars can reach decreases throughout the ride. It is important to note, that even though the energy leaves the system of cars, the energy has not disappeared. The energy has been transferred to objects or systems outside of the system of cars.

If work is done on a system or by a system, the amount and the types or forms of energy in the system may change. Adding energy to a system or removing energy from a system may be considered a transfer of energy. When energy stays within a system but changes into a different form, this is called energy transformation. Sometimes a small amount of energy may be added to a system to cause the potential energy of the system to transform into kinetic energy. Imagine a glass sitting on a table. The glass has gravitational potential energy. This potential energy will not transform into kinetic energy unless something happens, such as a person bumping the glass. The force on the glass is now unbalanced.

When the bump transfers a small amount of energy to the glass, it does work on the glass.

This energy now transforms into kinetic energy as the glass moves or falls.

A field is any region in which a noncontact force has an effect. Objects in a field may be acted upon by a force, depending on the type of field and the type of object. Magnets and magnetic objects in a magnetic field will be acted on by a magnetic force. Similarly, a mass in a gravitational field will be acted on by a gravitational force. Each object will have a different amount of potential energy, depending on its position in the field. The forces in the field will cause the object to move from a position of higher potential energy to a position of lower potential energy, unless another force prevents the object from moving.

Since the force of gravity is an attractive force, you have to apply force to masses to separate them. You are doing work on the masses, which transfers energy to the masses. If the masses are held in these same relative positions, the energy is stored as gravitational potential energy. When you release the masses, gravity pulls them together, transforming the potential energy into kinetic energy as they move together.

The potential energy of the system of masses decreases as they move closer to each other. Because the magnetic force between two magnets may be either repulsive or attractive, the change in potential energy of a magnetic system may vary differently.

Magnets are also an important part of many motors, which are used in a variety of devices, such as remote-controlled cars.

Unlike gravitational and magnetic potential energy, elastic potential energy does not depend on a field. Instead, the amount of elastic potential energy an object has depends on the state of the object itself. A catapult, a trampoline, and even an archer's bow all use elastic materials to store elastic potential energy. An elastic material is one that can return to its original state after being stretched or compressed. Springs and rubber bands are common examples of elastic materials. A force can be applied to an elastic material to stretch or compress the material. When the material is allowed to return to its original state, it can then apply a force and transfer energy to another object. This property can be used to do work and make toys and other objects move, bounce, and fly.