HPP Activity A32.v1
Some things never change
Exploration
Think about jumping vertically upwards.
GE 1.
1. What do you think happens to your kinetic energy as you make the jump?
2. What do you think happens to your gravitational potential energy as you
make the jump?
3. How do you think the sum of kinetic and potential energy changes
throughout the jump?


Set up the motion detector near the ceiling so that you can take some motion data while
jumping. Open the DataStudio file jump.ds.
Collect position and time data for a jump. Make sure you keep your arms at your sides
while you jump.
GE 2.
1. Look at the position versus time graph in DataStudio. What is position
range representing when the person is in the air (not in contact with the
ground)?
Create a velocity versus position graph in DataStudio by dragging the position
icon onto the time axis on the v-t graph. Copy this data onto the clipboard
(click in the graph, copy to clipboard); then paste in Excel.
2. Choose the point where the person leaves the ground as the reference level
for calculating gravitational potential energy. How can you use the position
measurements from DataStudio to calculate the potential energy of the
person?
3. Create columns in the spreadsheet for potential, kinetic, and the sum of
potential and kinetic energies. Create a graph of these three quantities as
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 A32.v1
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functions of position for the period where the person is not in contact with the
ground. Paste the graph here.
4. Describe the characteristics of the kinetic energy, potential energy, and sum
during the jump.
Invention
In a previous section we found that the work done by the gravitational force on an object does not
depend on the path taken. It just depends on the initial and final positions of the object. This
allowed us to define a special function, the potential energy function, that can be used to
calculate work done by gravity. Forces that have this characteristic, that the work they do does
not depend on the path taken, are called conservative forces.
The sum of kinetic and potential energies of an object is called the mechanical energy of the
object. We usually symbolize the mechanical energy by E.
E  K U
The exploration activity just completed suggests the following law, called the law of
conservation of mechanical energy.
If the only forces that do work on an object are conservative forces then the
mechanical energy remains constant.
Application
Locate the pole vault video clip and view it on your computer.
GE 3.
1. Describe qualitatively what you think happens to the kinetic energy and
potential energy of the person as he makes the vault.
2. Do you think mechanical energy should be conserved in the vault? Explain
your answer.
Let's find the actual mechanical energy right before the vaulter moves into the
air and when the vaulter is at the top.
3. What data must you collect to make these energy measurements?
Activity Guide
 2010 The Humanized Physics Project
HPP Activity A32.v1
4. Collect and record the necessary data.
5. Calculate the mechanical energy at the bottom and top.
Bottom:
Top:
6. Is mechanical energy conserved between these two points of the motion? If
not, try to devise a possible explanation.
Activity Guide
 2010 The Humanized Physics Project
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