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Energy
In physics, energy is the ability to do work. The movement produced by work changes an object's kinetic energy. When you pick up a bag of groceries from the kitchen floor and put it on the counter top you are performing work. You have put the bag in motion by increasing the kinetic energy of your muscles and some of that energy is converted to thermal energy by the molecules that make up your muscles.
Conservation of Energy
The first law of thermodynamics states that the amount of energy in a closed system is constant. This means that energy cannot be created or destroyed. However, energy changes into other forms all the time. For example, some of the kinetic energy of a rolling bicycle tire is converted to thermal energy when you engage the brakes and the rubber pads press against the tire. Heat did not cause the kinetic energy to change to thermal energy; heat was a byproduct of energy being used.
Thermal Energy and Heat
Your muscles convert some of the chemical energy stored in the food you eat to kinetic energy when you perform exercises, such as running, lifting weights and playing sports. Exercise makes you warm because the reactions that change chemical energy to kinetic energy give off excess energy in the form of heat. Heat is a method for transferring energy such as thermal energy which can be described as the kinetic energy of the molecules of a given substance. While heat does not cause the change from kinetic to thermal energy, its ability to transfer energy is a critical part of energy conversions.
Energy Conversion
As a form of potential energy, thermal energy can be converted into other kinds of energy, and kinetic energy can be converted into thermal energy. For example, when the Shuttle enters the Earth's atmosphere from orbit it has a great deal of kinetic energy. This kinetic energy is converted to thermal energy as the craft hits the atmosphere. Friction, not heat, caused the conversion from kinetic to thermal energy, but a lot of heat was generated as a byproduct.