LAB 1.5 A: POTENTIAL ENERGY AND HEIGHT
BACKGROUND:
In the last labs we have investigated kinetic energy, the energy of
moving objects. Do objects that are not moving have energy? Wile E.
Coyote (road-runnerus digestious) will push a boulder to the top of a cliff in hopes to destroy the
elusive Roadrunner (velocitus super-sonicus). Picture a boulder that sits at the edge of a high cliff.
If it falls on top of the Roadrunner that sits below, the rock would certainly do work on the
Roadrunner (ouch!). Energy is the ability to do work. The boulder had energy at the top of the cliff
because it had the potential to do work. This type of energy is called potential energy. Potential
energy is stored energy resulting from the position of an object. The boulder had a type of potential
energy called gravitational potential energy (GPE). In this lab we will investigate GPE using a
pendulum (See diagram below). Remember that energy can’t be measured until it does work. We
will infer the amount of GPE from the amount of work the pendulum does when it hits a block.
PURPOSE: To determine if height affects gravitational potential energy.
PRE-LAB QUESTIONS:
Use the sketch below and the background to answer the following questions.
Pendulum
Ruler
B
B
Block
A
1. a. Define potential energy.
b. At which position, A or B does the pendulum have gravitational GPE?
c. How will the pendulum get it’s GPE?
2. Refer back to the Wile E. Coyote situation in the background. Do you think the height the boulder falls
from makes a difference? Explain.
3. Scan the procedures. Briefly describe how we will test if height is a factor that affects GPE.
4. What will happen when the pendulum strikes the block? Use the terms work and energy transfer.
5. How will you know if release height affects the amount of GPE of the pendulum?
PROCEDURES: Part 1-Determining the Effect Of Height On GPE:
A. Copy the following data table:
Data Table #1: Effect of Height on Gravitational Potential Energy (GPE)
Distance block travels (Relative measure of work done)
Weight
Mass
Height
1st trial
2nd trial
3rd trial
Average distance
Large
5 cm
Large
10 cm
B. Bring a ring stand to your table. If necessary, loosen the screw of the metal ring on the ring stand and
slide the ring up to the top of the stand.
C. Acquire a large round weight with string attached. (If no string is attached, tie a string approximately
40 cm long to the weight.) Measure the mass of the large weight and record in the data table.
D. Wrap the string around the ring and adjust the length of string so that the weight hangs about one mm
above the table. (You can rotate the base of the ring stand so it does not interfere with the swing of the
pendulum.) Secure the string to the ring stand by wrapping the string around the ring numerous times.
Do not tie the string!
E. Set the block on the table so that the edge of the block rests against the weight when the weight hangs
motionless from the ring stand. (See position A in the diagram in Pre-lab.) Place a piece of tape on the
table so that its edge marks the starting location of the block.
F. Pull the weight straight back until the bottom of the weight is 5 cm above the table as measured by the
ruler. Have a group member hold the ring stand steady so it doesn’t move when the weight is released.
Release the weight so that it hits the center of the block and does the work of pushing it across the table.
You may need to do a few “practice” trials. Wait to record data until you get consistent results. It is
ideal for the block to move straight forward without twisting and/or spinning.
G. Measure and record the distance the block was pushed (work done). Perform three trials for this height.
Record the data and calculate the average.
H. Change the height. Double the release height to 10 cm and perform three more trials at this height.
Record the distance (work done) for each trial and calculate the average.
INTERPRETATIONS:
1. What is the Independent Variable in this lab?
2. What is the Dependent Variable in this lab?
3. What are the Controlled Variables in this lab?
4.
a.
b.
c.
Effect of height on potential energy.
Does height have an effect on the distance the block moved? Explain.
Does height have an effect on the work done by an object? Explain.
Does height have an effect on the potential energy an object has? Explain.
5. Is the relationship between height and potential energy direct or inverse? Explain.
6. Would you predict the relationship between height and potential energy to be linear or exponential?
Explain your reasoning. (Hint: you may want to review the kinetic energy lab results. Does double the
height result in double or quadruple the PE?)
CONCLUSION:
Wile E. Coyote is setting up a gigantic pendulum to try to kill the Roadrunner. He places a large log
across a canyon. In the center of the log, he suspends a long rope with a boulder tied to the end.
Provide Wile with some “expert advice” on the set-up of his pendulum. Where should he drop the
pendulum from and why? Use lab data to help support your explanation.