195A Lab: Impulse-Momentum
OBJECTIVES:
 Examine the relationship between impulse and change in momentum.
 Investigate conservation of momentum in an isolated system.
PART ONE: IMPULSE-MOMENTUM THEOREM
EQUIPMENT: Motion detector, D-link wireless USB device, wireless force probe
mounted on dynamics cart, LabPro interface, computer, Logger Pro software, track, cart
launcher at opposite end of track from motion detector, scale.
Note: For this experiment, “reverse direction” has been enabled so positive is towards
the motion detector.
COMPUTER SETUP:
 Insert Bluetooth device in back USB port, turn on computer, wait until computer
identifies Bluetooth, then open Logger Pro and scan for wireless devices from the
Experiment menu.
 Open the file L08A2-2 (Impulse and Momentum), from Additional Physics, etc.
 Change the sample rate to 1000 (ignore warning).
 Create a New Calculated Column… (found under the Data menu) to generate a
graph of momentum vs. time.
o Name: Momentum
o Short Name: p
o Units: kgm/s.
o Equation: Click on the variables button and choose “velocity”. Multiply
by the mass of your cart in kg. Your equation should look like this:
“velocity”* ###
 Click on y axis label of graph, and replace velocity axis with momentum axis.
TRACK AND CART SETUP:
 Attach the cart launcher is at one end, with an initial scale setting of 1.5 cm. There is
an easy-to-miss groove that has to be positioned correctly; ask for help if you don’t
see this feature.
 Set the motion detector is about 2/3 down the track.
 Attach the force probe to the cart, if necessary. The hook should make contact with
the center of the black rubber tip of the cart launcher when you are ready to take data.
DATA COLLECTION: Press Collect, and after the data collection starts, release the
cart, using the cart launcher. Make sure you see the impulse on the force vs. time graph
and the momentum immediately after the impulse, since friction will slow down the cart.
When you obtain a good graph of impulse and momentum, use the analysis features of
the software to display the following:
 value of impulse
 momentum immediately after collision (use Examine function)
 Title your graph “Impulse-Momentum” and Print with values displayed.
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1. Record your values of impulse (J) and ∆p, below, and find the % difference. They
should be within 5 % for maximum credit.
Impulse:
Momentum
Percent difference: |J-∆p| /J
x 100:_____________
2. Suppose we used a constant force instead of the variable spring force for the same
time interval. Calculate the value of that constant force if it were to provide the same
momentum change. Show your work:
Fconstant____________ N
3. What happens if there is an initial momentum? Roll the cart toward the cart launcher
with a velocity approximately the same as that of the 1.5 cm setting. Do not fire the cart
launcher this time; just use it as a wall. When you obtain a good graph of impulse and
momentum, PRINT your graph with the appropriate analysis displayed.
What is different about the way you will measure ∆p this time?
4. Compare the values of impulse (J) and ∆p. They should be within 5 %.
Impulse:
Momentum
Percent difference: |J-∆p| /J
x 100:_____________
When you have the two graphs with appropriate analysis printed:
 exit Logger Pro
 put away the cart with the wireless force probe and the Bluetooth link.
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PART TWO: CONSERVATION OF MOMENTUM
ADDITIONAL EQUIPMENT: LabPro interface with power supply, additional
motion detector, cables for motion detectors, 2 dynamics carts (one red, one blue) round
100 g masses.
TRACK AND CART SETUP: Put one motion detector at each end of the track.
COMPUTER SETUP:
 From Experiment menu in Logger Pro (you may have to go back up several
levels), choose Physics with Vernier.
 Choose file named “Momentum Energy” which may be either 18 or 19,
depending on version of software.
 Make a note of which motion detector is #1 and which is #2 and which cart they
are graphing. Note: one of the motion detectors has “reverse direction” enabled.
ZERO PROCEDURE: When using 2 motion detectors, you must zero them with the
carts. Put the carts at about the midpoint of the track, facing each other. Zero motion
detectors. You should only have to do this one time during the experiment.
INSTRUCTIONS Make the 2 cars undergo an inelastic collision (Velcro to Velcro) in
for several different scenarios, listed below (A-D). When pushing the carts, aim for a
velocity of approximately 0.5 m/s. Do this for the following four cases, adding needed
mass to the either cart when needed:
A. Two carts have equal mass, one is moving and one is stationary:
B. Cart 1 (stationary) has approximately twice the mass:
C. Cart 2 (moving) has approximately twice the mass:
D. Both carts moving in opposite directions, with one having approximately twice the
mass
1. Use the Examine features of the software to measure the momentum just before and
just after the collision. Choose your data just before and just after the collision.
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2. Fill in the data table below:
m1, m2 (kg)
p01
kg m/s
p02
kg m/s
pf
pf – p0
kg-m/s
A
B
C
D
3. Which collisions (A-D) showed conservation of momentum (consider momentum
conserved if (pf – p0) < 0.1 kg-m/s)? For any collision where momentum was not
conserved, explain why not:
4. State a general conclusion regarding conservation of momentum for inelastic
collisions e.g. always, never, in some cases (describe those cases):
5. Explain why the system of the 2 cars can be considered isolated. To do this, draw a
free body diagram of the system of the collision, showing all external forces (including
resistive rolling friction, if present). Then explain why each external force on your
diagram can be safely ignored for this experiment.
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