Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Force, Energy, and Power

It is easy to confuse the concepts of force, energy, and power, especially since they are synonyms in ordinary speech. The table on the following page may help to clear this up:

conceptual definitionA force is an interaction between two objects that causes a push or a pull. A force can be defined as anything that is capable of changing an object's state of motion. Heating an object, making it move faster, or increasing its distance from another object that is attracting it are all examples of things that would require fuel or physical effort. All these things can be quantified using a single scale of measurement, and we describe them all as forms of energy. Power is the rate at which energy is transformed from one form to another or transferred from one object to another.
operational definitionA spring scale can be used to measure force.If we define a unit of energy as the amount required to heat a certain amount of water by a 1 C, then we can measure any other quantity of energy by transferring it into heat in water and measuring the temperature increase.Measure the change in the amount of some form of energy possessed by an object, and divide by the amount of time required for the change to occur.
scalar or vector?vector - has a direction in space which is the direction in which it pulls or pushesscalar - has no direction in spacescalar - has no direction in space
unitnewtons (N)joules (J)watts (W) = joules/s
Can it run out? Does it cost money?No. I don't have to pay a monthly bill for the meganewtons of force required to hold up my house.Yes. We pay money for gasoline, electrical energy, batteries, etc., because they contain energy.More power means you are paying money at a higher rate. A 100-W lightbulb costs a certain number of cents per hour.
Can it be a property of an object?No. A force is a relationship between two interacting objects. A home-run baseball doesn't have force.Yes. What a home-run baseball has is kinetic energy, not force.Not really. A 100-W lightbulb doesn't have 100 W. 100 J/s is the rate at which it converts electrical energy into light.

Last Update: 2009-06-21