Download LAB AP1 Uniform Circular Motion

AP® Physics 1
Myers Park High School
Lab: Uniform Circular Motion
LAB: UNIFORM CIRCULAR MOTION
APPARATUS:
Hanging Bob
Centripetal Force Apparatus
Slotted Mass Set with Weight Hanger
Metric Ruler
Stop Watch
Triple Beam Balance
Rotating Shaft
Indicator Rod
BACKGROUND:
The purpose of this experiment is to observe a rotating object, use circular motion equations to calculate
the centripetal force acting on the object, and then attempt to measure the force directly by stretching the
spring to the same length it had while spinning the hanging bob.
First, you will rotate the apparatus so that the hanging bob travels in a circle passing over the indicator rod
(as shown above). Using the mass of the hanging bob, the period of revolution, and the radius of the
circular path, you will calculate the centripetal force that should be required to keep the object (the
hanging bob) in motion.
To check this value, you will try to measure the amount of force the spring exerts on the hanging bob
while it is rotating. Since there is no gauge or meter showing the force exerted by the spring, you will
measure the spring force by hanging weights over a pulley until the bob hangs at rest over the indicator
rod (as shown in part 2). When the bob is in equilibrium, the weight hanging over the pulley will be equal
to the spring force pulling back on the hanging bob.
PROCEDURE:
Part 1
1. Measure and record the mass of the rotating object (hanging bob). The triple-beam
balance measures in grams, to the nearest tenth of a gram. You will need to convert the
mass to kilograms before performing your calculations later in the lab.
2. Hang the object from the end of the support arm as shown in the diagram above, and
attach the spring between the object and the central shaft. It is best if the object hangs
vertically above the indicator rod at the height of the spring attachment before the spring
is attached, so that the string is vertical and the spring is horizontal when stretched. Use
the leveling screws (if necessary) to level the platform so that the object does not start
rotating when released, and adjust the counterweight (if necessary) so that the support
arm is roughly balanced over the central shaft.
3. Measure and record the radius of rotation from the center of the rotating shaft to the
indicator rod. Your ruler measures in centimeters, but you will need to convert the radius
to meters before performing the calculations below.
4. Rotate the shaft so that the hanging bob is rotating directly above the indicator rod.
Record the time to make 50 revolutions, keeping the object above the indicator rod each
time it passes. Repeat twice more for a total of three measurements, recording the time to
make 50 revolutions for each trial and then averaging your three times together.
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AP® Physics 1
Myers Park High School
Lab: Uniform Circular Motion
Part 2
Now, we will attempt to measure the actual
force the spring exerts on the hanging bob while
it rotates. Since we don’t have a gauge that
shows the force being exerted by the spring, we
have to do it by stretching the spring to the same
length it had while the bob was rotating. We’ll
keep the spring stretched by hanging weights
over a pulley so that the bob hangs in
equilibrium over the indicator rod. It is
important for the string connecting the bob to
the pulley to be horizontal so that the tension
corresponds to the horizontal force that kept the
bob in motion in Part 1.
5. Tie a line to the outer edge of the bob and tie the other end around a weight hanger.
Drape the line over the pulley and add weights from the slotted mass set to the weight
hanger until the spring stretches to the position shown above (with the bob directly above
the indicator rod).
6. Make sure the string is horizontal, or as close to horizontal as possible, before it passes
over the pulley. If necessary, adjust the height of the pulley so that the string pulling
outward on the mass is completely horizontal.
7. Record the total mass hanging over the pulley, including the mass of the weight hanger
itself, and convert this value to kilograms. Then, calculate the weight of the assembly
hanging over the pulley. Since the weight hanger is in equilibrium, the total weight
should be equal to the tension in the string, which will therefore match the spring force
pulling inward on the bob. This force represents the measured centripetal force acting on
the bob.
CALCULATIONS:
Finally, we will calculate the predicted centripetal force value based on the circular motion
equations.
8. Using the data from step 4, calculate the period (in seconds) and frequency (in hertz) of
the rotating object.
9. Calculate the linear speed of the object as it rotates (in meters per second).
10. Calculate the centripetal acceleration of the rotating object.
11. Calculate the centripetal force required to keep the object moving in a circular path. This
represents the calculated centripetal force acting on the bob.
12. Now that you have two measures of the centripetal force acting on the bob—one
measured directly by stretching the spring, and one calculated by observing the circular
motion of the bob—calculate the percent difference between your two values. How well
do your results support the theoretical relationship between centripetal force and circular
motion?
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AP® Physics 1
Myers Park High School
Lab: Uniform Circular Motion
Additional Questions:
1. Using the “top view” diagram above, draw three separate vectors to represent the
centripetal force, centripetal acceleration, and linear velocity of the rotating object at a
single instant.
2. What is a centripetal force? During the lab, what provided the centripetal force acting on
the rotating object?
3. Does the centripetal force acting on the mass cause it to change speeds? Explain why or
why not.
4. Sketch a “side view” free-body diagram showing all forces acting on the rotating object
as it passes over the indicator rod in Part 1 of the experiment.
 Are the forces on the object balanced or unbalanced during this part of the
experiment?
5. Sketch a “side view” free-body diagram showing all forces acting on the object as it
hangs over the indicator rod in Part 2 of the experiment.
 Are the forces on the object balanced or unbalanced during this part of the
experiment?
6. For an object moving in a circular path:
(a) How is the radius related to the centripetal force?
(b) How is velocity related to the centripetal force?
(c) How is mass related the centripetal force?
(d) If the mass and the radius are held constant and the period (T) is reduced to 1/2 of
its original value, how much does the centripetal force change from its original
value? Justify your answer.
Evaluation:
Write a conclusion that discuses and supports your results as they are related
to your objective. Also, suggest any improvements that could be made in the processes.
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AP® Physics 1
Myers Park High School
Lab: Uniform Circular Motion
Circular Motion lab Rubric
Title Block/Table of Contents (2)
Objective(s) Statement (2)
Data Collection
Measured Data Table (10)
Data Processing
Calculations (15)
Calculated Data Table
Additional Questions (18)
Summary/Conclusion (3)
50 Total Points
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