Work done per time is power

Work is a measure of energy. If I lift a book of mass m = 1 kg upward a distance of L = 1 m against the force of gravity, I need to produce

a total of 9.8 Joules of work. Fine.

But it takes the same amount of energy whether I lift the book quickly, in just 1 second, or if I very very sl-o-o-o-o-wly raise the book over 20 seconds. The same number of Joules, yes .... but something is different.

The difference there is in the RATE at which I do work. We have a name for the rate at which energy is used: power.



  Q:  What are the units of power?

Let's see how much power a person can produce. We'll run an experiment:

we'll find two volunteers, A and B, each of mass m = 60 kg
both volunteers will go up a flight of stairs, a distance H = 3.6 m. However,
- A will walk up the stairs in time T_A
- B will run up the stairs in time T_B



   Q:  How much work does A do?   
       How much work does B do?


   Q:  How much power does A produce?
       How much power does B produce?

After we've made the measurements and computed the power of our student volunteer, we can compare it to the power produced by a horse. You may have heard of a "horsepower" -- it is based on the amount of work that a horse could do over an entire day. In metric terms, one horsepower is about 746 Watts.



  Is either of our volunteers as powerful as one horse?