Energy
Energy is the capacity to do work and it exists in two basic forms: potential energy and kinetic energy. Potential energy is energy due to condition, position or composition. Kinetic energy is energy due to motion. Energy is not a material thing but rather a property of matter. Energy can be converted from one form to another.
Kinetic Energy
Kinetic energy is the energy associated with an object by virtue of its motion. The kinetic energy of an object of mass, m, moving with a velocity, v, has a kinetic energy Ek given by the equation
Ek = 1/2 mv2
This equation shows that the kinetic energy of an object depends on both its mass and its velocity. It also shows us that the velocity has a greater influence on kinetic energy than does mass. For example doubling the velocity of an object at a certain mass will quadruple its kinetic energy whereas doubling the mass would only double the kinetic energy.
Units of Energy
If we substitute the SI units of mass and velocity into the equation above we obtain the SI unit of energy, kg · m2/ s2. This unit is named the joule (J). Since the joule is an extremely small unit, kilojoules (kJ) are commonly used in chemistry. A non-SI unit of energy still used by chemists is the calorie (cal). The calorie was originally defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius. In 1925 the calorie was defined in terms of the joule: 1 cal = 4.184 J.
Potential Energy
Potential energy, as we stated above is energy due to position or configuration. For example if we lift an object to some height above the surface of the earth, it will have potential energy. We arbitrarily assign zero potential energy to objects on the earth's surface. The formula for the potential energy of an object of mass, m, at a height, h, above the earth's surface is Ep = mgh, where g is the acceleration of gravity.