Centripetal Force Lab
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
Plastic tube
Nylon cord (about 1.0m long)
1 stopper with holes
Weight set
Stopwatch
Meter stick
Tape or paper clip
Procedure
Cut a length of cord 1.0m long. Fasten one end of the nylon
cord securely to the rubber stopper. Pass the other end
through the glass tube and fasten a 50g hooked mass with an
extra 50g mass (total 100g) to it. Adjust the cord so that there
is about 0.5 m of cord between the top of the tube and the
stopper. Attach a paper clip,tape 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 tape/paper clip stays just
below the bottom of the tube. Do not let the clip rub on the
bottom of the tube. The radius of the stopper should remain
constant during this investigation. 0.5m would be a good
radius. Make several trial runs before recording any data.
Measure what your radius is and place in the data table.
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
Centripetal Force Lab
Name_________________________
Group Members__________________________________________________________
Data
Trial
1
2
3
4
5
6
Calculations
Hanging
Mass
(kg)
Mass of
Stopper
(kg)
Total
Time
(s)
Radius
(keep
constant!!)
(m)
Centripetal
Force (Fc)
(N)
Period
Circumference
Speed
(s)
(m)
(m/s)
0.10
0.15
0.20
0.25
0.30
0.35
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.
2. Find the period of revolution by dividing the total
time by the number of revolutions.
Period = Total time / 20 revs
4. Calculate the circumference of revolution from the
radius.
Circumference = 2 x 3.1416 x radius
5. Use the circumference and period to find the speed.
Speed = Circumference / Period
Graph Plotting Using Graphical Analysis and 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. Analyze your data using a squared fit and include the fit equation on your graph. Submit with this
handout.
Analysis Questions
1. What provides the centripetal force needed to keep the stopper moving in a circle?
a) the stopper b) the weight c) neither of these
2. What direction is this force pointing in?
a) towards the outside b) along the tangent c) towards the center
Although your graph should show this if you follow the appropriate procedure correctly, you should answer these question
according to theory!
3. If you double the speed of revolution of the stopper, what happens to the force needed to keep the stopper moving
around?
a) one half
b) doubles
c) one fourth d) quardruples
4. If you half the speed of revolution how much would you expect the force to change by?
a) one half
b) doubles
c) one fourth d) quardruples
5. If instead the speed ( along with the mass) was kept constant and the radius was allowed to double, the centripetal
force would to change by
a) one half
b) doubles
c) one fourth d) quardruples
6. With the graph forced to a squared plot, what should the value of the constant be? Show your calculation below.
7. Determine the percent error in the theoretical value of the constant with the actual experimental value. (mass/radius)