Name _________________________________ Due Date _____________Period ___________________
Circular Motion Lab Activity
Prelab Questions:
1) Write the following equations from your notes:
a) Equation for the speed of an object the is moving in a circle
b) Equation for the centripetal acceleration
c) Equation for the force needed to keep an object moving in a circle (this is the centripetal force)
2) Suppose someone was whirling a ball in a horizontal circle on a string.
a) What real force is supplying the centripetal force?
b)
If the person were to increase the mass of the ball, what would happen to the centripetal force
needed? (Hint: Look at your equation in 1c)
c) If the person were to increase the speed at which the ball travels, what would happen to the
centripetal force needed?
Activity 1
Watch the teacher demonstration of whirling a ball on a string, record the data and answer the questions.
Changing the Mass of the Ball
1) We are going to test the prediction you made in 2b) above.
a) What two variables do we need to keep constant in order for this to be a valid experiment?
b) In the following data table, we will record the time it takes to go around once, just to see if we are
staying consistent. We will also record the mass of the ball and the tension force.
Tension force in the
Time for 5
Time for one
Mass of the ball in string (this supplies
cycles in
period
kg
the centripetal
seconds
in seconds
force) in Newtons
c) Compare the tension force for two different masses when the periods were similar. What is the
effect of increasing mass on the tension force?
d) Was your prediction about the relationship between mass and tension force from the prelab
correct?
e) Is the relationship between mass and tension force directly proportional or inversely proportional?
f)
Why is it important to compare the effects of masses only when the periods are similar?
g) Why did we average 5 cycles to get the period, instead of just using one cycle?
Changing the Speed
2) We are going to test the prediction you made in 2c) above.
a) What two variables do we need to keep constant in order for this to be a valid experiment?
b) In the following data table, we will record the radius, the period and the tension force. We will
then calculate the speed using v=2πr/T.
Tension force in the
Time for 5
Time for one
Radius in
string (this supplies
cycles in
period
Speed in m/s
meters
the centripetal force)
seconds
in seconds
in Newtons
c) Compare the tension force for two different speeds. What is the effect of increasing speed on the
tension force?
d) Was your prediction about the relationship between speed and tension force from the prelab
correct?
Activity 2
In this activity you and your group will change the centripetal force applied to a rubber stopper (by adding
washers), keeping the radius and mass the same, and determine how the period, average speed and
centripetal acceleration change.
Procedure:
On each lab table there is a glass tube wrapped in tape, a string with a rubber stopper attached to one end, the
string passes through the glass tube and there is a bent paperclip attached to the other end of the string. You
will also have 18 metal washers, an alligator clip, a stopwatch and a meter stick.
1) Extend the string out on the table and measure 60 centimeters from the middle of the rubber stopper
to the beginning of the glass tube. Place the alligator clip on the string at the end of the glass tube to
hold the radius constant when you begin spinning the rubber stopper.
2) Place three metal washers on the paperclip. Hold the glass tube and begin spinning the rubber
stopper around your head. CAUTION: MAKE SURE NO ONE IS IN THE PATH OF THE RUBBER STOPPER
BEFORE YOU BEGIN SPINNING.
3) Spin the stopper slowly at first and gradually increase speed. You will notice that the metal washers
slowly will rise. Increase speed until the alligator clip is right at the bottom of the glass tube but not
touching.
4) Have a student use the stopwatch to time ten full revolutions of the stopper. Record the time for ten
revolution in the table below.
5) Add three more washers and repeat the experiment. Repeat until you have performed the experiment
with 18 washers on the paper clip adding 3 each time.
6) Now calculate the period (one revolution), average speed and acceleration for each trial.
Radius: 60 cm = 0.60 m
Number of
Washers
Time for 10
Revolution
(seconds)
Period
(Seconds)
Average Speed
(m/s)
Centripetal
Acceleration
(m/s2)
3
6
9
12
15
18
Questions:
1) By adding more washers to the paperclip you increased the amount weight hanging on the string.
Describe how the number of washers is related to the centripetal force acting on the stopper.
2) Describe how the period of the stopper changed as more washers were added to the paperclip.
3) Describe how the average speed of the stopper changed as more washers were added to the
paperclip.
4) Describe how the centripetal acceleration of the stopper changed as more washers were added to the
paperclip.
Activity 3
Use the following to answer problems (1-6)
A ball is attached to a string and the string is fed down a PVC pipe and then attached to a force sensor. The
distance from the center of the ball to the beginning of the PVC pipe is 70 cm. A student then holds the PVC
pipe and swings the ball around in a circle above his/her head. The period of the ball moving around the
student’s head is determined with a stopwatch and the force sensor captures the forces required to swing the
ball at the measured period. These numbers are placed in the table below. The experiment is performed
again but this time the student swings the ball faster. The data is collected and recorded in the table. The
experiment is performed four more times and the data recorded.
Average
Centripetal
Centripetal
Period (s)
Speed
Acceleration
Force (N)
(m/s)
(m/s2)
2.0
1.3
4.0
0.95
6.0
0.78
8.0
0.68
10.0
0.60
12.0
0.54
1) Calculate the average speed of the ball for each period and record your answers in the table above.
2) Calculate the centripetal acceleration for each period and record your answer in the table above.
3) Now make a Force (Y-axis) vs. Period (X-axis) graph. Make sure you have decent scales and have
labeled your axes for each graph. Draw a best fit smooth curve through your data.
4)
4) What kind of relationship exists between the centripetal force acting on the ball and the period?
a) Linear
b) Quadratic
c) Inverse
d) No relationship
5) Now make a Force (Y-axis) vs. Acceleration (X-axis) graph. Make sure you have decent scales and have
labeled your axes for each graph. Draw a best fit line through your data.
6)
6) The slope of the Force-Acceleration graph is equal to the mass of the ball. Determine the mass of the
ball in (kg) by finding the slope of the graph. Show your work and record your answer below.
7) A student is running in a circle on a flat track with a radius 20 meters. If the student completes a lap
every 15.4 seconds:
a) Find the average speed of the student around the track in m/s.
b) Find the magnitude of the centripetal acceleration of the student in m/s2.
c) What object is responsible for supplying the force that is causing the centripetal forces acting on
the student?
d) What kind of force (normal, gravitational, spring, tension, kinetic friction, or static friction) is
responsible for the centripetal force acting on the student?
e) If the student has a weight of 765 N, find the magnitude of the centripetal force acting on the
student in newtons.
Extra Credit:
f)
If the coefficient of static friction between the student’s shoes and the track is 0.75, find the
maximum speed the student can travel around the track and not slip.
Extra Credit:
g) Find the period the student would travel around the track at the maximum speed found in part (f).
Extra Credit:
h) The student comes back to the same track the next day after it has rained and notices that he/she
cannot run faster than 5.4 m/s around the track without slipping. Find the coefficient of static
friction between the student’s shoes and the wet track.