Studio Physics I
Impulse and Momentum Change
The impulse, J = ∫F dt , is a quantity that combines
the net force and the time interval over which the force acts.
J = p = pf – pi
A) Start LoggerPro. Then go File, Open, <Your Physics1 Data Folder>, L08A2-2 (Imp
& Momentum).mbl .
B) Once you have opened the correct file, the next thing that you must do is to calibrate
your force probe. This will require making two probe readings with two different and
known forces applied to the force probe. The forces are applied to the probe through
the hook ATTACHED TO THE FORCE PROBE. Follow the steps below to carry
out the calibration:
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Click “Set-up” at the top of the computer screen.
Click “Sensors”.
Click on the F with the DIN1 below it.
Click the “Calibrate” tab above the F.
Click on “Perform now” off to the right. (If you don’t have “perform now”
showing, you did not make a connection to the ULI when opening the file. You will
have to completely close LoggerPro and reopen the file.)
Remove the hanging mass from the force probe if it is attached. There is now zero
force on the probe.
Enter zero in the box (replacing the –5) for the force.
Click “keep”.
Hook the string for the hanging mass on the hook of the force probe.
While holding the cart still, run the string over the pulley and let the mass hang down,
pulling on the force probe. The force applied to the probe is now the weight of the
mass. That is, (0.052 kg)(9.8 m/s2) = 0.51 N.
While still holding the cart with hanging mass attached, enter 0.51 (N) in the second
box that has appeared on the right (replacing the 5) for the force.
Click “keep”.
The calibration process is complete. Click on OK to close the calibration box.
C) Check to make sure that the calibration process has been successful. Failing to
do this may result is a complete waste of time doing everything that follows.
Read the force displayed on the bottom of the screen while the hanging mass pulls on
the stationary cart. The force should be about 0.5. Remove the string attached to the
hanging mass from the force probe. The force should now be close to 0. If both of
these readings check out, all is well. If you are having trouble, ask for help.
D) The experiment file has been set up to record force and motion data at 50 data points
per second. A pull on the force probe will be recorded as a positive force, and
velocity away from the motion detector is a positive velocity.
E) Be sure your track is level. Use one of the carpenter’s levels and the leveling
screw at the end of the track.
Rev. 2003 Bedrosian
1. Remove the hanging mass from the force probe. You will not use it again unless
you need to recalibrate your force probe for some reason. Place the cart at least 0.5
meters away from the motion detector. Click the collect button to begin taking data.
When you hear the motion detector start clicking, give the cart a short pull on the hook of
the force probe, and then let the cart move freely. While you are collecting data, be sure
to hold the cord to the force probe up off of the table (that is, don’t let the cord drag or
pull on the cart), and keep your hand farther away than the cart from the motion detector.
Sketch the actual velocity versus time and force versus time graphs on your paper. Write
down the approximate value of the maximum force, the time period over which the force
was applied and the velocity of the cart after the pull.
2. Examine the graphs carefully. Discuss how the time interval over which the force is
applied compares to the time interval over which the velocity changes. Does the velocity
change in time periods during which there is no force applied to the cart? Discuss your
results and answer to the question in terms of the impulse applied to the cart and the
change in the cart’s momentum.
3. Use the analysis and statistics features in the software to measure the average
velocity of the cart before and after the pull. Don’t forget to include a sign. ( To get the
average velocity from the graph, use your mouse to mark the region of interest, then go to
analyze, then statistics and say ok). What is the average velocity after the pull? What is
the average velocity before the pull?
4. Calculate the change in momentum of the cart. Show your calculations. The mass of
the cart is 500 grams and the mass of the force probe is about 165 grams.
5. Use the integration routine in the software to find the area under the force-time
graph. This is the impulse in N-s. (To do this, use your mouse to mark the region of
interest, go to analyze, then integral and say ok.) Did the calculated change in
momentum of the cart (question above) equal the measured impulse applied to cart
during the pull? What is the percent difference?
6. Add masses to your cart to make the total mass twice about twice as large. (Each
black bar is 495 grams.) What is the new mass of your cart and force probe? Zero the
force probe by clicking on the zero button at the top of the screen. Then click collect,
wait for the motion detector to start clicking and then give the cart the same pull (to the
best of your ability). Be sure to keep the force probe cord from dragging and keep your
hand out of the way of the motion detector. Try several times or until you get about the
same force and duration as for your initial try above in step 1.
7. Find the average velocity, as you did before and calculate the change in momentum.
Find the impulse as before. Is the impulse now about the same as before or is it different
by a factor of 2 (the change in the mass)? Is the change in momentum about the same as
before or is it different by a factor of 2 ? What is different by about a factor of 2?
Rev. 2003 Bedrosian