HPP Activity A31.v1
Let’s get moving
Exploration
Let’s go back to our human-sized cart rolling down a ramp. We will try to relate the work done
by gravity as it goes down the ramp to the final speed of the cart. Pick a person to ride the cart.
GE 1.
1. Measure the mass of the cart and person.
2. We need an easy, quick way to measure the final speed at the bottom. One
method is to measure it directly with a motion detector. Another method is to
measure the time it takes to go down the ramp. How could you use a time
measurement to find the final instantaneous velocity? Hint: think about your
kinematic equations at constant acceleration.
3. Collect data on work done by gravity and final speed for at least 4 ramp
heights.
Final speed [m/s]
H [m]
Work [J]
0
4. Graph the work done as a function of v2. Paste it hear.
5. What is the shape of the curve?
6. What is the equation that best fits the data?
7. Try to relate this equation to the mass of the cart and person.
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 A31.v1
Invention
We saw that the function mgy was related to work done by gravity, and we called it the
gravitational potential energy. Here, we see that the work done by gravity can be related to the
final speed of a falling object through the quantity 1/2mv2 . So, it is sensible to think of this as
an energy, energy of motion. We define this to be kinetic energy.
The above exploration suggests that the net work done on an object is related to the change in
kinetic energy. This relationship is called the work-energy theorem.
Wnet  K
Note: this equation is strictly true only for point particles or rigid (non-deformable) objects.
Application
View the digital video file shotput.mov. Your task is to estimate the average force of the hand on
the shot.
GE 2.
1. Describe your plan for estimating the average hand force.
2. Collect appropriate data.
3. What is the average hand force?
Activity Guide
 2010 The Humanized Physics Project
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