Physics Activity

More Power!

Purpose:

To estimate the power output by various muscle groups of your body.

Equipment:

meter stick

scale

timer (watch or stopwatch)

Discussion:

Power is the rate that work is done, and when you exercise you do work. For instance, each time you do a pushup, you lift the weight of your body several centimeters, and the work that you do equals the force that you exert (= your weight) times the distance that you lift yourself. Many common exercises involve doing work. In this activity, you will measure distances, forces, and times in order to calculate the work and power output of your muscles.

Procedure:

Choose an exercise that involves doing work (such as lifting an object, raising on toes, sit-ups, push-ups, etc.).
Perform this exercise slowly, and:
- measure or estimate the force that you exert during the exercise. (Hint: At the surface of the earth, 1 pound = 4.45 Newtons (approximately))
- have your lab partners help you measure the distance that you exerted the force.
- enter your measurements in a data table.
Now, perform several repetitions of the exercise as fast as you can (while being safe!). Have your lab partners count the repetitions and measure the total time it takes.
Repeat for some other exercise. If your first exercise involved your arm muscles, pick another exercise that involves your legs, or vice versa.

Calculations:

The work required to perform your exercise = the force you exerted times the distance you exerted it.
The total work you did = the work for one repetition times the number of repetitions that you performed.
Your power output = the total work you did divided by the total time taken.

For example, suppose your exercise is lifting a book which weighs about 10 Newtons. Suppose you lift the book 1 meter, 10 times in 20 seconds. Then:

Work in 1 repetition = (Force)(distance) = (10 N)(1 m) = 10 J
Total work done = (Work in 1 rep)(# of reps) = (10 J)(10) = 100 J

Power output = (Total work)/(time) = 100 J / 20 s = 5 Watts

Analysis:

How does the power output of your arm muscles compare to the power output of your leg muscles? Why do you think this is?
How does your power output compare to the power output of a 60 Watt light bulb? A small lawnmower engine (1 horsepower = 746 Watts, approximately)? Is the power output of your muscles more or less than you thought it would be?

last update May 24, 2000 by Jerry L Stanbrough