Experiment
20
Impulse and Momentum
The application of a force to a body will result in a change in motion for the body. This change
is displayed by an observed acceleration.
Force Sensor
For this experiment, a cart will be pushed along a smooth surface with an applied force for a
certain period of time and the cart will exhibit a change in momentum due to this impulse. The
force applied is measured by a Force Sensor. The cart velocity throughout the motion is
measured with a Motion Detector. Logger Pro will then be used to find the average force during
the time interval and the cart’s change in velocity.
Cart
and force sensor
Motion Detector
rosneS ecroF
Motion
Detector
droc citsalE
Elastic cord
Lab Table
OBJECTIVES
Measure a cart’s momentum change and compare to the impulse it receives.
 Compare average and peak forces in impulses.
 Identify the sources of error in the experiment.

PRE-LAB QUESTIONS:

Define Momentum-

What are the Units for Momentum?

How is Momentum Calculated?

How will a change in velocity affect the momentum of a body?

How does a force affect the velocity of a body?

Which would have a greater effect on the velocity, a peak force value or an average force
value?

Can a force change momentum? Explain.
Experiment 20
PROCEDURE
1. Measure the mass of your dynamics cart and the force sensor. Record the value in your lab
notebook. Explain how you were able to use the force sensor for this measurement.
_______________ kg
2. Measure the mass of two black bars. Record the value in your lab notebook.
_______________ kg
3. Connect the Motion Detector to DIG/SONIC 1 of the interface. Connect the Force Sensor to
Channel 1 of the interface. If your Force Sensor has a range switch, set it to 10 N.
4. Open the file “19 Impulse and Momentum” in the Physics with Vernier folder. Logger Pro
will plot the cart’s position and velocity vs. time, as well as the force applied by the Force
Sensor vs. time.
5. Place the cart on the track.
Place two bar masses in the cart and tape the force sensor to the top of the bars in a secure
manor.
6. Place the Motion Detector one the end of the lab table and have it face toward the wall.
Make sure there are no obstructions in the path of the sensor and it can read the position of
the cart. The sensor should be placed to allow the cart to travel toward it without being
interrupted.
7. Click
, select Force Sensor from the list, and click
to zero the Force Sensor.
8. Practice pushing the cart by pressing on the force sensor. Take care to not be detected by
the motion detector. This will take some practice and require you to focus on the task at
hand. You will want to cause the cart to accelerate and reach a constant velocity.
Practice pushing the cart with a smooth motion to keep it in contact with your hand and
traveling toward the motion detector. The push should be enough to have the cart
accelerate toward the motion detector but still reach a constant speed before the
detector stops reading the cart after you let go. Remember, the lab is going to study the
motion of the cart from rest.
9. Click
to take data; push on the cart and confirm that the Motion Detector detects the
cart throughout its travel.
Inspect the force data. If the peak exceeds 10 N, then the applied force is too large. Push the
cart with a lower force. If the velocity graph goes flat while pushing the cart, then you have
pushed the cart for too long and need to push the cart for less time.
10. Once you have made a run with good position, velocity, and force graphs, analyze your data.
The graphs should clearly show the cart accelerating and the force you applied to the force
sensor.
Using the graphs:
Choose the interval on the velocity vs. time graph corresponding to
a time when you were pushing on the cart and then let go. The speed becomes constant as
the cart is no longer being pushed. When the speed becomes constant, that is the end of your
interval.
Drag the mouse pointer across this interval. Click the Statistics button, , and read the
average velocity. Record the value for the initial velocity and final velocity of the cart for the
time interval in your lab notebook.
Impulse and Momentum
11.
Now record the time interval of the force. On the force vs. time graph, drag across the graph,
capturing the entire period when the force was non-zero and the velocity was increasing.
Find the average value of the force by clicking the Statistics button, , and also read the
length of the time interval over which your average force is calculated.
*** The number of points used in the average divided by the data rate of 50 Hz
gives the time interval t. ******
Record the values in your data table.
12. It is time to gain other very valuable information. From the force vs. time graph, select the
region over which the force acts for the prior interval.
Now, click the Integrate button, , to find the area under this curved portion of the graph.
Record it in your lab notebook. Print the Graph of your data with all of the analysis
tables.
13. Repeat the lab for two more different forces. Push the cart with two different amounts of force
and record the data in your lab notebook.
DATA TABLE
Mass of cart + sensors + masses
Trial
Final Velocity
vf
(m/s)
Initial Velocity
vi
(m/s)
kg
Average
Force
F
(N)
Duration of
Force
t
(s)
Value of the
Integral of
Area under
force time
graph
(N*s)
1
2
DATA ANALYSIS – SHOW SAMPLE WORK FOR EACH CALCULATION
DONE
1. From the mass of the cart and change in velocity, determine the change in momentum as
a result of the force acting on the cart. Make this calculation for each trial and enter the
values in the second data table. Show your work below for at least one calculation.
Change in momentum = (mass) (change in velocity)
2. Determine the impulse for each trial from the average force and time interval values.
Record these values in your data table. J = F t
Multiply the average force times the time interval. Show work below.
Experiment 20
DATA ANALYSIS CHART –
Do Not Complete until you follow each step in the Data Analysis.
Trial
Impulse
Ft
(Ns)
Change in
momentum
(kgm /s)
1
2
3. Look at and analyze the impulse and change in momentum values for the trials above.
Are the two columns different or are they similar? What is the difference in the two
values? Are the units different?
CONCLUSION – Answer in your lab notebook.
1. Look at the shape of the last force vs. time graph. Is the peak value of the force significantly
different from the average force?
Which force value, Peak or Average, is larger?
Is there a way the same impulse could be experienced with a much smaller force?
Explain how that could happen.
2. How close were your values for change in momentum and impulse? What does the
comparison of the values tell you about the change in momentum for a body and the Impulse
experienced during that change?
3. The integral value on your data sheet is the area under the force time graph or the
region between the graph and the x-axis. How does this value compare to the impulse and
change in momentum calculated for the same time interval? Are the Impulse and change in
momentum connected in any way as shown by your data?