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....

Gravitational PE converted directly into heat

A 50-kg firefighter slides down a 5-m pole at constant velocity. How much heat is produced?

Since she slides down at constant velocity, there is no change in KE. Heat and gravitational PE are the only forms of energy that change. Ignoring plus and minus signs, the gravitational force on her body equals mg, and the amount of energy transformed is

(mg)(5 m) = 2500 J .

On physical grounds, we know that there must have been an increase (positive change) in the heat energy in her hands and in the flagpole.

Last Update: 2009-06-21