In 30 seconds and with a force of 750 N, a body builder lifts some heavy weights above the ground by 2 meters. How much power does the body builder have?

When you lift a box up off the floor, you must exert a force at least equal to the weight of the box. If you lift the box at constant speed, you "do work" on the box that would be equal to the force you exert (the weight of the box) times the height through which you lifted the box. If you exert a force of 50 Newtons on the box and lift it 1 meter high, then you did (50 N) x (1 m) or 50 N-m of work on the object. 1 Newton-meter is called a Joule (J), pronounced "jewel." In lifting the 50 N box upward 1 meter at constant speed, you would do 50 Joules of work on the box. That means your body used 50 J of energy to lift the box.

Equation for Work: Work = Force x distance, or W = F d.

Units for Work: N-m (Newton times meter, pronounced "Newton meter") or the Joule (J).

What does work produce? When work is done on an object, the energy of the object changes. In the example above, the amount of energy used to lift the box is stored as gravitational potential energy in the box (see below). If you push a car that has run out of gas, the work you would do in pushing the car would be converted into the energy of motion known as kinetic energy. Therefore, we conclude that work produces a change in energy (either potential or kinetic or both).