Power Problems
Example. I used up 2000 J of energy walking up a flight of stairs. It took me 10 s. What was my power? Answer. P = E/t = 2000 / 10 = 200 W.
Example. A light bulb gives off 10 J of light energy every second. However, it needs 100 W of electrical power. What is its efficiency? Answer. Useful power is the part that produces light, that is 10 / 1 = 10 W. So the efficiency is 10 / 100 = 0.1, or 10%.
Example. I push a car with a force of 100 N. The car start from rest
and reaches a final velocity of 1 m/s.
Find the initial and final power.
Answer. Initial power is P = Fv = 100 x 0 = 0 W.
Final power is P = Fv = 100 x 1 = 100 W.
Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 21 September 2010.