# Centripetal Force Lab

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```					Name: ____________________________________                                           Date: ________
Physics K / Integrated Science                                                       Per: _________

CENTRIPETAL FORCE LAB
Objectives:
- show that a square relationship exists between speed and force for uniform circular
motion
- relate experimental results to the equation Fc = mv2 / r

Materials:
- Centripetal Force Apparatus (glass tube w/ rubber stopper on a string)
- 50 g mass hanger                             - assorted slotted masses
- meterstick                                   - stopwatch

Procedure:
1. Get the Centripetal Force Apparatus from your teacher.

2. Ensure that there is a piece of tape in the shape of a flag on the string below the glass tube.

3. Pull the rubber stopper until the tape flag hits the glass tube, but does not get stuck inside.

4. Measure the length of the string from the glass tube to the middle of the rubber stopper.
When you spin the stopper, this will be the radius of the circle. Record this as the radius in

5. Place the 50 g mass hanger on the loop at the end of the string.

6. Spin the rubber stopper in a horizontal circle over you head so that it pulls the mass hanger
upward. Spin just fast enough that the flag is just barely below the bottom of the glass tube,
but does not fall downward or get pulled into the tube. See the diagram below. Hint: It is
easiest to start the twirling by holding the hanger so the circle has a small radius and then

hanging mass

7. While one person is twirling the stopper, another person should use a stopwatch to measure
the time for 20 revolutions. Record the time in your data table.

8. Repeat Steps 6 & 7 four more times, adding 50 g to the mass hanger each time, up to a total
mass of 250 g (including the hanger). Record all data in your data table.
Data:

Radius (m) =_____            Circumference (m) = ______

Hanging     Weight of     Fcent (N)   Time for 20      Period (s)    Velocity       Velocity 2
Mass       Hanging                   Revolutions                     (m/s)          (m/s)2
(kg)       Mass (N)                      (s)
0            0            0              0              0             0               0

0.050

0.100

0.150

0.200

0.250

Analysis:

below, including equations and units. Enter the circumference in the data table.

2. Calculate the weight (Fgrav) of the hanging mass for each trial. Show a sample calculation
below, including equations and units. Enter the weight in the data table.

3. Use the times you measured to calculate the period for each trial. Show a sample calculation
below, including equations and units. Enter the period in the data table.

4. Calculate the velocity of the rubber stopper for each trial. Show a sample calculation below,
including equations and units. Enter the velocity in the data table.

5. Calculate the square of the velocity (velocity2) for each trial. This is NOT the centripetal
acceleration. Just take the velocity and square it. Show a sample calculation below and enter
your results in the data table.
6. Draw free body diagrams for both the hanging mass and the stopper. Assume the stopper is
twirled in a circle at a constant speed and the hanging mass is not accelerating up or down.

rubber stopper                          hanging mass

7. Explain why there is a net force of zero on the hanging mass. What does the weight of the
hanging mass need to equal?

8. Explain why there is a non-zero net force on the rubber stopper.

9. What real force produces the net force on the rubber stopper? _____________________
Since this force always pulls toward the center of the circle, we can call this _____________.

10. Record the value of the centripetal force in the data table for each trial. Hint: See #7.

11. Use LoggerPro to create a plot of Fcent vs. Velocity. Click on the Curve Fit button to create a
best-fit line using the “Proportional” fit. Print your graph, including a title and axis labels.

12. Use LoggerPro to create a plot of Fcent vs. Velocity2. Click on the Curve Fit button to create
a best-fit line using the “Proportional” fit. Print your graph, including a title and axis labels.

13. Which of these two graphs have data that is best represented by the “Proportional” best-fit
line? ________________________________

14. In conclusion, the centripetal force on an object is proportional to the _______________.

15. In the diagram below, draw the direction of the velocity vector (v), the acceleration vector
(a), and the centripetal force vector (F) for a rubber stopper in circular motion.

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