"Work is the transfer of energy."

Work is done on an object when you transfer energy to that object. For introductory thinking, this is the best definition of work.

If you put energy into an object, then you do work on that object.

If a first object is the agent that gives energy to a second object, then the first object does work on the second object. The energy goes from the first object into the second object. At first we will say that if an object is standing still, and you get it moving, then you have put energy into that object.

For example, a golfer uses a club and gets a stationary golf ball moving when he or she hits the ball. The club does work on the golf ball as it strikes the ball. Energy leaves the club and enters the ball. This is a transfer of energy. Thus, we say that the club did work on the ball.

And, before the ball was struck, the golfer did work on the club. The club was initially standing still, and the golfer got it moving when he or she swung the club.

Formula For Work

In almost all cases considered when studying mechanical forms of energy, when work is done on an object a force is applied to the object, and the object is displaced while this force is acting upon it. That is, the object moves as a result of a force being placed on it.

In the previous golf example the club places a force on the ball, and this force acts on the ball over the short distance through which the club and the ball are in contact as the ball is being hit. Energy is transferred as the force acts over this displacement.

The amount of work is calculated by multiplying the force times the displacement. That formula looks like this:

W = F * d

At first we will consider only forces that are aimed in the same direction as the displacement. For example, we will imagine an object being pushed horizontally to the right, and the object will be moving horizontally to the right as a result of this applied force.

Conservation of energy
To scientists, conservation of energy is not saving energy. The law of conservation of energy says that energy is neither created nor destroyed. When we use energy, it doesn’t disappear. We change it from one form of energy into another. A car engine burns gasoline, converting the chemical energy in gasoline into mechanical energy. Solar cells change radiant energy into electrical energy. Energy changes form, but the total amount of energy in the universe stays the same. Scientists at the Department of Energy think they have discovered a mysterious new form of energy called "dark energy" that is actually causing the universe to grow!

Energy Efficiency
Energy efficiency is the amount of useful energy you get from a system. A perfect, energy-efficient machine would change all the energy put in it into useful work—an impossible dream. Converting one form of energy into another form always involves a loss of usable energy. In fact, most energy transformations are not very efficient. The human body is a good example. Your body is like a machine, and the fuel for your machine is food. Food gives you the energy to move, breathe, and think. But your body isn’t very efficient at converting food into useful work. Your body is less than five percent efficient most of the time. The rest of the energy is lost as heat. You can really feel that heat when you exercise!

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