Centripetal Force and Speed Lab
Name: _____________
period: ___
(uses “Centripetal Force Apparatus # 14005” from the Science Source)
Animated video available at: (if you don’t want to actually do it)
https://www.thephysicsaviary.com/Physics/Programs/Labs/ClassicCircularForceLab/index.html
This lab simulation will let you determine the speed needed to keep an object in circular motion. You will
be able to change the force holding the object in a circle by clicking on the washers (each washer is 10
grams). You can adjust the radius of the circle by clicking on the masking tape that is just below the tube.
You can also change ...
Here is a good example video of doing the lab (3 minutes):
https://www.youtube.com/watch?v=3Wyq0HmlXSU
The purpose of this lab is to investigate the relationship between the speed of an object in
uniform circular motion (UCM) and the centripetal force on the object.
Equipment:
centripetal force apparatus washers or weights
meter stick
stopwatch
data & analysis sheet
Discussion:
A force which pulls an object toward the center of a circle is called a centripetal force. How
much centripetal force needs to be exerted to cause an object to move in a circle? Your
experience should tell you that the amount of centripetal force that you need to exert depends on
a. the mass of the object you are whirling - heavier objects require more force,
b. how fast you are whirling it - going faster requires more force, and
c. the radius of the circle smaller radius of circle requires more force.
The textbook says (page 176 of Conceptual Physics):
Fcentripetal =
m v2
r
; v2/r is the centripetal acceleration; v is the velocity and r is the radius
but how can we verify the quantitative relationship between centripetal force and mass, speed,
and radius? This question can't be answered all at once, since a scientific experiment is designed
to vary one quantity (holding all others constant) and measure its effect on one other quantity.
The most difficult quantity (of mass, speed, and radius) to hold constant from trial to trial in an
experiment is the speed of the object, so it is easiest to study the effect of speed on centripetal
force, since it is relatively easy to hold the mass of the object and the radius of the circle
constant.
You will use an apparatus similar to the one pictured above to measure the effect of speed on
centripetal force. You can hold the mass constant during a set of trials by always whirling the
same object. You can keep the radius of the circle constant (with a little practice) by keeping the
upper clip a fixed distance below the glass tube while whirling the object.
Procedure:
1. Place a small number of weights or washers (be sure that all of the washers you use are
the same size.) on the bottom clip of the apparatus. This part of the apparatus hangs
straight down, and the weight of the washers supplies the centripetal force.
To save time each group will be assigned a number of washers to report to the class in
case some groups don’t get all the data collected; our group is responsible for ____
washers
2. Set the upper clip so that the rubber stopper will have a radius of .6 meters. Practice
whirling the stopper (or ball) until you can keep the top clip a short distance below the
bottom of the glass tube while the stopper whirls. IMPORTANT! If the clip touches the
bottom of the glass tube, the weights are no longer supplying the centripetal force! If
the clip rises or falls appreciably as the stopper whirls, the radius of the circle is
changing. Practice!
3. Use a stopwatch to measure the time taken for 10 revolutions (this is designed to decrease
the human error in timing short intervals of time). The data table allows you to repeat
three times. Record your data.
4. Change the number of washers on the bottom clip (centripetal force) and repeat steps 3
and 4. Repeat for several different weights. Record the data.
5. If time permits: Change the position of the top clip to change the radius of the circle.
Repeat the experiment for this new radius. Be sure to indicate where the radius changes
in a new/revised data table.
6. If you have time, you might try to determine the relationship between mass and
centripetal force. In order to do this, you need to keep both the radius of the circle and the
speed constant while you vary the mass and the centripetal force. You can design your
own data table for this. You could also investigate the relationship between the radius and
the centripetal force.
Results:
1. Calculate the period of revolution, T (the time to go around once) for each trial. Show a
sample calculation.
2. Calculate the linear speed (the circumference of the circle divided by the time to go
around), v, of the stopper for each trial. Include a sample calculation.
Note:
v = _2r__
T
3. There is a column in the data table for the calculated centripetal force; this should be the
same as the weight of the wathers
4. Theoretically, the centripetal force should be directly proportional to the square of the
speed. To check this, add a column to your data table for v2. Construct a graph of
centripetal force versus v2. Remember that it is customary to put the quantity you change
(force, in this case) on the horizontal axis, and the quantity that changes by itself (speed)
on the vertical axis. Be sure that you pick the largest convenient scale for your graph and
draw the best smooth curve through your data points.
Conclusion: (can be written on the back of the graph you construct)
Is the graph of centripetal force versus speed squared a straight line? So, what can you say about
the relationship between centripetal force and speed, then?
Each washer has a mass, m, of _.00586_ kg;
the mass of the stopper, M, is _0.0145_ kg
The radius of our circle, r, was 0.6 m (make sure you don’t twirl so fast the radius is held in
check by the clip and not the weight of the washers)
(Note: Bold headings have to be measured; italicized column headings have to be calculated)
# of
washers
3
6
9
12
15
18
21
Force =
W = mg
(N)
time for
10 Rev
(sec)
Time
Period
Sec)
Avg
Velocity
2 r / T
(m/sec)
Velocity2
(m2/sec2)
Calculated
Centripetal
Force
M v2 / r
(N)