HPP Activity A33.v2
Horsing around: Defining Power
Exploration
GE 1.
1. Take a mass and set it on the floor next to your lab table. Have one of your
lab members lift the mass very, very slowly and place it on the table. How
much work did you do?
2. Now lift the mass as quickly as you can and set it on the table. How does
the amount of work you did this time compare to what you did in step one?
3. What was different about the two trials?
4. Does the work you do depend on the time it was done over? Explain.
Invention
GE 2.
1. Why might it be useful to consider the time it takes to lift the mass and set it
on the table?
2. Write an equation to determine the rate of doing work.
3. This quantity is called power. Substitute the SI base units into the formula
and determined what the standard SI unit for this quantity would be.
Application
We are all familiar with the term horsepower, but what exactly this term means and how it relates
to you may not be so clear. In order to give you a better idea of just how much power this is, we
are going to figure out how much power you can generate.
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity A33.v2
GE 3.
1. Make a prediction about the amount of horsepower a member of your group
can generate over a short period of time. Record your prediction.
2. Now we need to find the rate of work that person can do. Let’s use a flight
of stairs in this building. Calculate the amount of work that person will do
climbing that flight of stairs. You may need to take a measurement. Show
your calculations.
3. Now find the time it takes that person to climb the stairs. Caution: this is
not a race, make sure you climb the stairs safely and under control. Calculate
the amount of power generated.
4.What should the label be on your answer? Convert your answer into
horsepower.
5. How did your actual horsepower compare with the predicted value you
made earlier?
6. What are some limiting factors that affect your power output?
Exploration
Imagine pulling a block across the table at a constant speed of 40. [cm/s].
GE 4.
1. If you were to pull the block at a constant 60. [cm/s] do you think the force
required would be larger?
2. Try the experiment. You do not have to pull at 40 and 60 [cm/s], just use a
slow speed and a larger speed. How does the required pulling force compare?
3. How does the work done in pulling the block 1.0 [m] compare for the slow
and fast cases?
Activity Guide
 2010 The Humanized Physics Project
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HPP Activity A33.v2
4. How does the power produced compare for the two cases?
Invention
If you exert the same force on an object but move at different speeds, the power required
changes. You must do the same amount of work in a smaller amount of time if you are pushing
at a faster speed. If F is the component of force in the direction of motion and v is the magnitude
of velocity in that direction then
P  Fv
Application
We will investigate how much power is required to push someone around on the human-sized
cart.
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Find a volunteer to sit on the human-sized cart and another volunteer to push the personcart system.
Turn on a moderate amount of friction.
Set up the computer to measure the velocity of the cart and the applied force using the
force plate.
The person sitting on the cart should hold the force plate by the handles. The pusher
should push against the force plate and push the cart towards the motion detector. Be
careful not to run into the motion detector.
Repeat until you get a good set of force and velocity measurements.
GE 5.
1. Paste a graph below showing the force and velocity as functions of time.
2. Get DataStudio to calculate the power used on the cart-person system as a
function of time. Explain how you do this here.
3. Paste a graph below of the power used as a function of time.
Activity Guide
 2010 The Humanized Physics Project
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