Center of Mass Physical Lab
Purpose: This lab is intended to demonstrate and gain experience with using Newton’s second law for
rotation to find the center of mass for discrete and continuous masses.
Required Materials:
Safety goggles – wear them throughout this lab.
1 – Twelve inch ruler
1 – pencil
20 - pennies
Answer all questions and fill in data table with a font color other than black.
Deliverables:
1) Photograph of your lab set up for one trial.
2) Data table with showing torque versus distance from center of mass
3) Answers to discussion questions at the end of the lab.
Procedure:
1) Lay a pencil on a flat surface. This will serve as the fulcrum for your experiment.
2) Set the center of the ruler on the pencil so that the length of the ruler is perpendicular to the pencil,
3) Place stack of 3 pennies onto one end of the ruler. Record the distance from the center of the ruler
that these pennies were placed.
Initial placement : ____________________cm
4) Predict the distance from the fulcrum that you must place the center of mass of 4, 5, 6, 7 and 8 penny
stacks to balance the ruler.
Show a sample calculation of your prediction for 7 pennies here:
After you have made a prediction for each of the stacks, balance the ruler with each of the stacks and
record your data and calculations below.
Trial
1
2
3
4
Penny
Stack
Height
(# of
pennies)
4
5
6
7
Calculate distance from
fulcrum of the 4,5,6,7
stacks (cm)
Measured distance from
fulcrum of the 4,5,6,7
stacks (cm)
Error between calculate
and measure locations
(%)
5) With 3 pennies at the end of the ruler, move the fulcrum to the 4 inches from the 3 stack of pennies.
Add pennies to the far end of the ruler until the ruler is able to lift the 3 pennies. After the ruler tilts,
remove one penny from the opposite stack. Slide this stack of pennies toward the fulcrum until the
balances with the original 3 pennies. Assume the mass of a penny is 2.50 grams.
Using the distances of the pennies from the fulcrum, estimate the mass of the ruler. Show your
calculation of this estimate here:
Discussion / Conclusions:
1) In steps 1-4 with the fulcrum at the center, did you need to take the mass of the ruler into account?
Why or why not?
2) For steps 1-4, graph force versus distance from the fulcrum. Include the graph in a separate excel file.
Is this a linear graph?
3) Write an equation using Newton’s second law for rotation that supports you conclusion in (2).
4) What was the central consideration you needed to introduce when you moved the fulcrum from the
center of the ruler to the 4 inch mark?