Centripetal Force Lab
Name/Date
Objectives
In this practical activity you are going to
verify the mass of a rotating stopper using
your knowledge of constant speed circular
motion.
Introduction
Things in life are often changing direction.
This can be taken both metaphorically and
literally. For example, what might seem a
bind right now might tomorrow be a source
of interest because of its application in a
profession or your personal life.
Things like roller coasters, the planets,
galaxies, centrifuges and even your
everyday washing machine or waste
disposal unit, have got one thing in
common, they all contain parts that
change their motion over time.
There are a number of ways to change the
motion of something. The object could
change the magnitude of its motion but
retain its direction, or it could retain its
magnitude but change its direction of
motion or... it could do both. This sounds
confusing doesn't it? Anyway, with all
these permutations, a force is needed to
cause such a change in motion.
Preliminary Questions
1. Explain why is circular motion at
constant speed not a natural state of
motion (according to Galileo).
2. What do we formally call a change in
motion per unit time and which of
Newton's laws adequately expresses its
relationship with a net force?
3. What do you call a force that is directed
towards the center of circular motion?
Apparatus
Glass tube
Nylon cord (about 1.5m long)
2 stoppers with holes (varying mass)
Weight set
Stopwatch
Meter stick
Procedure
Cut a length of cord 1.75m long. Fasten one end of the
nylon cord securely to the rubber stopper. Pass the
other end through the glass tube and fasten a 100g
mass to it. Adjust the cord so that there is about 1.0m of
cord between the top of the tube and the stopper.
Attach a crocodile clip or marker to the cord just below
the bottom of the tube.
Support the 100g mass with one hand and hold the
glass tube with the other. Whirl the stopper by moving
the tube in a circular motion. Slowly release the 100g
mass and adjust the speed of the stopper so that the
crocodile clip stays just below the bottom of the tube.
Do not let the clip rub on the bottom of the tube.
Make several trial runs before recording any data.
When you have learned how to keep the speed of the
stopper and the position of the crocodile clip relatively
constant, have a classmate measure with the
stopwatch, the time required for 20 revolutions. Record
this time. Stop the whirling of the stopper, place the
apparatus on the top of the lab table with the cord
extended the way it was during the experiment (as
indicated by the position of the crocodile clip), and
measure the distance from the center of the glass tube
to the center of the rubber stopper. Record this distance
in the data table as r. Record the mass of the stopper.
Repeat the procedure for Trials 2 -6. Keep the radius
the same as in trial 1 and use the same rubber stopper,
but increase the hanging mass at the end of the cord.
For Trial 2 use 150g, increasing each time by 50g up to
350g.
Data and Calculations Tables
Data
Trial
Calculations
Hanging
Mass
(kg)
1
0.10
2
0.15
3
0.20
4
0.25
5
0.30
6
0.35
Mass of
Stopper
(kg)
Total
Time
(s)
Radius
(m)
Centripetal
Force (Fc)
(N)
Period
Circumference
Speed
(s)
(m)
(m/s)
Example Calculations
Show the calculations for Trial 1 in the spaces provided below. Enter the results of the calculations in the
appropriate spaces above.
1. Calculate the weight (Fc) of the hanging mass and enter in the table as the centripetal force.
Fc = weight of hanging mass = hanging mass x 10 N/kg
2. Find the period of revolution by dividing the total time by the number of revolutions.
Period = Total time / 20 revs
3. Calculate the circumference of revolution from the radius.
3. Use the circumference and period to find the speed.
Circumference = 2 x
3.1416
Speed = Circumference / Period
x
radius
Graph Plotting
1. Using the graph area, plot a fully labeled graph for Trials 1 - 6 putting velocity on the x-axis and
centripetal force (Fc) on the y-axis. Put the origin at (0,0) with an appropriate scale on each axis. Include a
title, axis labels and units. What type of equation most fits your graph?
Analysis Questions
1. What provides the centripetal force needed to keep the stopper moving in a circle?
2. What direction is this force pointing in?
3. According to your graph, if you increase the speed of revolution of the stopper, what happens
to the force needed to keep the stopper moving around?
4. Based on your graph If you double the speed of revolution would you expect the centripetal
force needed to double? If not, how much would it change?
Graph Area:
Application:
5. What provides the centripetal force needed for a car to go around a circular off-ramp at
constant speed?
6. What will happen if a car tries to go around a circular off-ramp curve at a speed which requires
a centripetal force greater than what the road can provide?
7. If a car doubles its speed on a circular off-ramp how much more centripetal force will be
required for it to make the turn?