# Kinetic energy

Kinetic energy is the energy an object has because of its motion.

###### About kinetic energy
• Objects move or change direction when a force is applied to them.
• Planets moving through space, cars, people and atoms all contain kinetic energy.
• The application of force by one object on another results in kinetic energy being transferred between them.
• When one object strikes another, it is the force of the impact that allows kinetic energy to be transferred.
• Every interaction between objects involves a force being applied to both and kinetic energy being transferred.
• The term force is used to describe the energetic interaction between objects and is used regardless of scale.
• Galaxies, human beings and electrons all apply forces to the things around them.
• Energetic interactions that result in the transfer of kinetic energy between objects is described as involving work.
• Only motionless objects at a temperature of absolute zero are incapable of interacting with one another and so incapable of doing work.
• At an atomic scale:
• Electrical energy passes through a circuit as electrons collide with one another transferring their kinetic energy.
• Heat is produced as photons strike an object, transferring their kinetic energy to electrons within the molecules on its surface.
• The human senses of sight, hearing and touch are tuned to respond to different forms of kinetic energy when a force is applied to them.
• When a person hears a sound or sees something, that is evidence of work having been done.
###### An example

A person pushes a heavy boulder up a hill. Each time they push they apply force to make the boulder move. By the time they reach the top they feel exhausted because the work involved has used up a lot of energy, some of which has been transferred to the boulder. As soon as the boulder is released its kinetic energy is transferred to other boulders as it crashes back down the hill.

If the observer in this example is too heavy and doesn’t move, no energy is transferred to it. Work is still done but this time the energy is transferred between the person’s muscles etc. and released into the air in the form heat.