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:
 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 N/kg) = 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. 2004 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, 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 before the pull? (Hint: Easy answer.) What is the average velocity after 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?
Exercise
A mountain climber is sliding down a vertical rope. Her total mass, counting equipment, is 65 kg. By
adjusting the friction on the rope she controls the tension force that the rope exerts on her during a twosecond interval. The tension on the rope (magnitude) as a function of time is shown in the figure below.
She begins at rest at t = 0. Find her speed at t = 2 seconds. Use g = 9.8 N/kg and neglect air resistance.
6. (a) Identify the forces on the climber. (Hint:
There are two.) (b) Select a coordinate system;
we recommend +X in the direction of
acceleration. (c) Draw a free-body diagram
showing the directions of +X and acceleration.
(d) Determine whether each force is positive or
negative in your coordinate system. (e) Write
Newton’s 2nd Law based on your diagram.
7. Draw (not sketch) a graph of total or net
force versus time. (This is the left side of the
equation in 6e.) Your graph should show the
shape of the curve, have a scale, and clearly
identify minimum and maximum points.
Positive/negative on your graph should agree
with your choice of coordinate system.
Tension (N)
637
312
time (s)
0
8. Find the integral of your graph and use that
0
with the impulse-momentum theorem to find the
speed of the climber at the end of the two-second interval.
Rev. 2004 Bedrosian
0.2
2