This document describes an experiment to investigate the relationship between force and velocity of an object moving in a circular path. Students will use a string, rubber stopper, washers, and timer to rotate the stopper in a circle with varying masses attached. They will measure the time it takes for 20 revolutions and calculate the period for different numbers of washers. By plotting force versus the inverse of period squared, students can determine if centripetal force is inversely proportional to the period squared as predicted.

1. Why are there holes
in the side of a
washing machine?

2. Uniform Circular Motion
Round, and
round, and
round we go!!!

3. Purpose
To investigate the relationship between force
and velocity of an object moving in a circle with
a fixed radius.

4. Hypothesis
Predict how you think the size of a radius will
affect the speed at which an object will move in
a circle.

5. Materials
•
•
•
•
•
•
•
string (1.5-2.0m),
rubber stopper,
timer,
straw or pen cylinder,
clip,
10 large washers,
1 large paperclip

6. Procedure
• Assemble your apparatus so the rubber stopper
tied to one end of the string will rotate in a circle
having a radius of 1 m.
• Begin with 6 washers tied to the other end of the
string.
• While rotating the stopper at constant
velocity, measure the amount of time required
for 20 cycles and record in a data table.
• Repeat this process adding 2 more washers (8
total) and then again for 2 more washers (10
total).

7. Period (T)
• The period is the amount of time needed to
complete one revolution.
• Calculate and record the period for each
amount of washers using the time needed to
complete 20 revolutions.

8. Radial Force
• Determine the total mass of the washers.
• Convert this mass to kilograms (kg).
• Calculate the radial force by determining the
weight of the washers.
– The downward weight of the washers is equal to
the radial force exerted on the rubber stopper.

9. Data Analysis
# of
Time for
washers 20 rev
6
8
10
Period (T)
Mass (kg)
Radial
Force
1/T2

10. Graph it!
• Plot a graph of Force (F) versus 1/T2.
• What kind of relationship do these two
quantities have?
• Centripetal force is inversely proportional to
the square of the period (T), does this graph
verify this relationship?

11. Force
Radial Force
1/T2

12. Check the slope!
• Calculate the slope of the plot.
• The slope should equal a value close to
m(4π2R) where m is the mass of the stopper
and R is the radius of its path.
• Does this verify the equation F = (m4π2R) / T2

13. (R.E.R.U.N.) CONCLUSION:
Recall what you did during this lab
Explain why you did this lab and what you were trying to find out
Reflect on the lab’s meaning. If you predicted, did it match your
hypothesis? Was your hypothesis wrong? Why?
Uncertainty (errors that were in the lab that you could not control
[aka: weather problems, measurement mistakes, etc.] or errors
that you came across during the lab that you could fix for the
next time you perform this lab)
New questions or new discoveries (normally you put at least three)