CENTRIPETAL FORCE LAB
NAME: Meghan Brown
Honors Physics – 11
Background: An object moving with changing speed in the same direction is undergoing
acceleration. If an object moves with a constant speed but in a changing direction, it is also
undergoing acceleration. Both types of accelerations require a net force. This acceleration that
causes a change in direction is called “centripetal acceleration”, and the force producing it is
called the “centripetal force”. The equation relating centripetal force, mass, radius of the circle
and velocity is: Fc = (m v2) / r
Objective: After completing this experiment, you should be able to verify the relationship
between centripetal force, mass, velocity, and the radius of orbit for a body that is undergoing
centripetal acceleration.
Equipment: wrapped glass or plastic tube, nylon cord, several rubber stoppers of different sizes
(must share!), varying slotted masses and mass hanger OR hanging masses, stopwatch,
meterstick, tapes.
Procedure:
1. Obtain a rubber stopper (use a # 6, 7, or 8 stopper for this section). Measure the mass of the
stopper and record this in your data table. (The number on the stopper is the SIZE of the stopper,
NOT its mass!)
2. Fasten one end of the nylon cord securely to the rubber stopper. Pass the other end of the cord
through the tube.
3. Fasten a 100-gram mass to the other end of the cord (if you are using a hanger and slotted
masses, don’t forget to include the mass of the hanger).
4. Adjust the cord so that there is about 0.50 meters of cord between the top of the tube and the
stopper. (This is the radius of the circle we are creating.) Record this radius in your data table.
5. Place a piece of tape on the cord just BELOW the bottom of the tube and above the mass
hanger. This is to allow you to keep track of the radius of the circle you are creating and keep it
constant.
6. Support the 100 g mass with one hand and hold the tube in the other. Whirl the stopper by
moving the tube in a circular motion horizontally above your head. Slowly release the 100 g mass
and adjust your swinging of the stopper so that the tape stays just below the bottom of the tube
and does not touch the tube. Practice this several times until you can keep the motion of the
stopper fairly constant. b7. When you have learned how to keep the velocity of the stopper and
the position of the tape fairly constant, have your partner measure the time it takes for the stopper
to complete 20 revolutions.
8. Record this time in the data table in the data & calculations spreadsheet.
9. Repeat this procedure for trials 2 thru 5 with the same rubber stopper and radius, but increasing
the mass at the end of the cord by 50 g each trial. Record all data.
10. For trials 6-10, keep the same stopper and place 100 g on the end again. Change the radius of
your circle each trial. Start with 0.3 m and increase to around 1 m. (The increments in this section
do not matter.)
11. For trials 11-15, place the radius at 0.50 m again, keep 200 g on the end, and vary the size of
the rubber stoppers used. To get a wide range of mass for the stoppers, you may want to tie more
than one stopper together on the end. Record all data.
12. Complete the calculation section and answer all questions.
CALCULATIONS:
1. Calculate the weight of the hanging mass and enter this as the centripetal force.
2. Find the period of one revolution (time to make one circle) by dividing the total time by the
number of rotations made (total time/20).
3. Calculate the circumference of revolution using the radius of the circle.
4. Use the circumference as distance and the period as the time, and calculate the velocity using
d = vt.
5. *Create a graph for trials 1 – 5 using centripetal force as the x – axis and velocity as the y –
axis.
6. *Create a graph for trials 1 & 6-10 using the radius as the x-axis and the velocity as the y-axis.
7. *Create a graph for trials 11-15 using the mass of the stopper as the x-axis and the velocity as
the y-axis. *Include these graphs in your lab report. They must be created using Microsoft Excel.
A linear trendline must be included on each graph and the equation must be displayed.
This lab report must be typed and submitted through blackboard. The entire report must be
submitted as a single Microsoft Word file (version 2003 or earlier). This means that you must
copy the data chart, calculation chart, and graphs into your lab report.
QUESTIONS:
1. Draw a free body diagram of the forces on the stopper. Draw a separate free body diagram for
the forces on the hanging mass. Explain using these diagrams what is actually causing the
centripetal force on the stopper to keep it in the circle (relate the stopper to the hanging mass).
Fc
Tension
Tension
Fgk
2. At any point of the rotation of the stopper, if you were to let go or the string were to break, in
what direction relative to the radius of the circle would the stopper go flying? Explain why in
terms of Newton’s 1st and 2nd laws of motion.
-1st Law = an object in motion stays in motion unless acted upon by another force. The
stopper will keep moving until it hits something else stops it, or until the gravity pulls it
down.
-2nd Law = Is in effect while the stopper is swirling in the air, so when the stopper is stopped
being swung the 2nd law isn’t in effect.
3. Based on the first graph (velocity vs. centripetal force), as the centripetal force went up, what
happened to your velocity?
-The velocity made a steep incline. It kept increasing, then it levels off.
4. According to your second graph, what happens to the velocity of the object when the radius of
the circle increases?
-The Velocity increases.
5. According to the third graph, what happens to the velocity of the object as the mass of the
object moving increases?
-The velocity decreases.
6. How would the centripetal force need to change if your radius was increasing, but you wanted
to keep the velocity and mass of the object that was swinging the same?
-Fc = mv2 / r
The radius is in the denominator, so as the denominator grows larger, the quotient will get
smaller.
DATA:
Varying
Stopper
Varying
Radius
Varying
Mass
Data
Trial
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Hanging Mass
(g)
100
150
200
250
300
100
100
100
100
100
200
200
200
200
200
Mass of Stopper
(g)
12.8
12.8
12.8
12.8
12.8
12.8
12.8
12.8
12.8
12.8
7.7
12.8
28.5
47.9
72.5
Total Time
(s)
20
18.2
9.37
7.66
7.51
7.15
10.06
10.53
12.28
13.07
6.25
7.93
9.16
10.31
11.66
Radius (m)
0.5
0.5
0.5
0.5
0.5
0.3
0.5
0.7
0.9
1
0.5
0.5
0.5
0.5
0.5
Calculations
Trial #
Centripetal
Force (N)
Period (s)
Circumfrence
(m)
Velocity
(m/s)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0.98
1.47
1.96
2.45
2.94
0.98
0.98
0.98
0.98
0.98
1.96
1.96
1.96
1.96
1.96
1.00
0.91
0.47
0.38
0.38
0.36
0.50
0.53
0.61
0.65
0.31
0.40
0.46
0.52
0.58
3.14
3.14
3.14
3.14
3.14
1.88
3.14
4.40
5.65
6.28
3.14
3.14
3.14
3.14
3.14
3.14
3.45
6.70
8.20
8.36
5.27
6.24
8.35
9.21
9.61
10.05
7.92
6.86
6.09
5.39
CF
0.98
1.47
1.96
2.45
2.94
Velocity
3.14
3.45
6.70
8.20
8.36
Radius
0.3
0.5
0.7
0.9
1
Velocity
5.27
6.24
8.35
9.21
9.61
S. Mass
7.7
12.8
28.5
Velocity
10.05
7.92
6.86
47.9
72.5
Velocity
Table 1
10.00
8.00
6.00
Velocity
4.00
2.00
0.00
0.00
0.50
2.00
1.50
1.00
3.00
2.50
3.50
Velocity
Table 2
15.00
10.00
Velocity
5.00
0.00
0
0.5
1
1.5
Velocity
Table 3
12.00
10.00
8.00
6.00
4.00
2.00
0.00
Velocity
0
20
40
60
80
6.09
5.39