Physics 221
Pre-lab assignment: Write the names, title, objective, relevant equations, and the beginning of
the equipment sections. You will be writing the procedure as you perform the lab, as well as
updating the equipment list and taking data. Your notebook will be checked off during lab, and
the photocopy of the pages is due on Thursday, February 4.
Your full name
Your partners’ names
Lab 4: Finding the tension force of a cart on a ramp
Objectives:
• Calibrating a digital force sensor
• Measuring the force exerted by a stationary cart on a ramp at five different angles
• Comparing the measured value of the force to the theoretical value of the force
Introduction and relevant equations:
Newton’s second law states that the force is proportional to both the mass of the object the force
is impressed upon and the consequent acceleration of the object.
(Draw a free-body diagram showing all of the forces acting on the cart. Label all of the forces
with the appropriate vector).
(Further, set up an appropriate coordinate axes to analyze the net force acting on this cart, and
then write the two equations that show the forces acting in the x and y directions).
Equipment:
• collision cart with force sensor attachment
• Vernier Dual Range Force Sensor
(Complete the list of equipment, using brand or manufacturer names where appropriate)
(Photograph or sketch of the setup. Label the various parts of the apparatus.)
Procedure:
Calibrating the force sensor
(Write the steps you used to calibrate the force sensor. Note that you should have the force sensor
setting on “10 N”. Consider how you would show that a force of 9.81 N was acting on the
sensor?)
Measuring the force exerted by the cart
(Examine the photographs below and mimic the setup; note that some of the equipment you will
use is different. Write the steps you used to measure the force exerted by the cart at five different
ramp angles. The last run should be at a 90° inclination. The force sensor setting should still be on
“10 N”. One big hint: instead of our usual rule about “1 in the last digit”, use the “play” function
on the datalogger to record 10 force measurements over a few seconds, then calculate the mean
and standard deviation (which is a better measure of uncertainty anyway).)
Data and preliminary analysis:
(What masses must be weighed and recorded?) (What is the uncertainty in the mass?)
(Set up a data table like the one below)
Run
number
Angle of
incline (°)
1
2
3
4
5
90°
Measured force on cart by
string (N)
Mean
Std. dev.
Theoretical
force on cart by
string and its
uncertainty (N)
Percent error
Reality check: How many significant figures should each column have, including the mean and
standard deviations?
Analysis and results:
Calculate the force that should be exerted by the string on the cart (the theoretical force), using
the free body diagram set up in the “Relevant equations” section. Fill in the appropriate column
on the previous table.
Show one of these calculations in this section of the notebook. Clearly state the value of g you are
using.
Assume the value of g has an uncertainty of “1” in its last digit, then calculate the uncertainty in
the theoretical value of the force. Show the setup of this calculation for one of the rows.
Calculate the percent error between your mean measured force and the theoretical force. Show
one of these calculations clearly.
Discussion
1. Why did the string that connects the cart and the top of the ramp need to be parallel to the ramp
surface? In other words, what free-body diagram-related assumption was satisfied by making the
string and the ramp surface parallel?
2. Why were “10 force measurements over a few seconds” taken for each run? Hint: there are two
reasons.
Conclusion
3. Suppose someone claims that we did not take into account ramp friction, and thus friction is the
most significant source of error in this experiment. How do the results of your final run support or
contradict this claim?
4. Were the “theory” predictions of force consistent with the measured force? How can you tell?