Name: _____________________________
Date: ___________
Block: ___________
Impulse and Momentum Lab
BACKGROUND
The impulse-momentum theorem relates impulse and the change in momentum of an object. An impulse is when a
force acts over time on an object. The relationship between impulse and momentum can be described with an
equation:
Impulse = Change in Momentum
F  t  mv f  mvi
For this experiment, a dynamics cart will roll along a level track. Its momentum will change as it reaches the end of an of
a long rubber band, much like a horizontal bungee jump. The rubber band will stretch and apply a force until the cart
stops. The cart then changes direction and the rubber band will soon go slack. The force applied by the cord is measured
by the force sensor on the wireless dynamics system. The cart velocity throughout the motion is measured with a
Motion Detector. Using the LabQuest 2 to find the average force during a time interval, you can test the impulsemomentum theorem.
Motion Detector
Force Sensor
Elastic cord
MATERIALS


LabQuest 2
Vernier Motion Detector


dynamics cart and track setup
Wireless Dynamics System (WDSS)
PRELIMANARY QUESTIONS
1. In a car collision, the driver’s body must change speed from a high value to zero. This is true whether or
not an airbag is used, so why use an airbag? How does it reduce injuries?
2. You want to close an open door by throwing either a 400 g lump of clay or a 400 g rubber ball toward it.
You can throw either object with the same speed, but they are different in that the rubber ball bounces
off the door while the clay just sticks to the door. Which projectile will apply the larger impulse to the
door and be more likely to close it?
OBSERVATION DATA TABLE
ANALYSIS TABLE
Mass of cart
Trial
Elastic 1
kg
Final
Velocity
vf
Initial
Velocity
vi
Average
Force
F
Duration
of Impulse
t
(m/s)
(m/s)
(N)
(s)
Trial
Elastic 1
1
1
2
2
Elastic 2
Elastic 2
1
1
2
2
Impulse
Ft
Change in
momentum
(Ns)
(kgm /s) or
(Ns)
% difference
between Impulse
and Change in
momentum
(Ns)
ANALYSIS
1. Determine the impulse for each trial from the average force and time interval values. Record these values in
your data table.
2. From the mass of the cart and the final and initial velocities, determine the change in momentum the car
experienced from the rubber bands. Make this calculation for each trial and enter the values in the second data
table.
If the impulse-momentum theorem is correct, the change in momentum will equal the impulse for each trial. The
instruments may have measurement errors, along with friction and shifting of the track or Force Sensor, will keep
the two from being exactly the same. One way to compare the two is to find their percentage difference.
3. Use the equation below to calculate the percent difference between your experimental values.
% 𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 =
𝐿𝑎𝑟𝑔𝑒𝑟 𝑉𝑎𝑙𝑢𝑒 − 𝑆𝑚𝑎𝑙𝑙𝑒𝑟 𝑉𝑎𝑙𝑢𝑒
× 100
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑉𝑎𝑙𝑢𝑒
4. Do your data support the impulse-momentum theorem? _______________________________________
5. Look at the shape of the last force vs. time graph.
a. Is the peak value of the force significantly different from the average force?
______________________________________________________________________________
b. Is there a way you could deliver the same impulse with a much smaller force?
______________________________________________________________________________
6. When you use different elastic materials, what changes occurred in the shapes of the graphs?
______________________________________________________________________________
a. Is there a cause and effect relationship between the type of material and the shape? _________
7. When you used a stiffer or tighter elastic material, what effect did this have on the duration of the impulse?
______________________________________________________________________________
a. What affect did this have on the maximum size of the force? ______________________________
b. Can you develop a general rule from these observations? _________________________________
________________________________________________________________________________
PROCEDURE – Check off each step as you do them to ensure you are properly doing the lab.
1.
Measure the mass of your dynamics cart and record the value in the data table. Make sure you record it in Kilograms.
2.
Turn on the WDSS and use the Bluetooth to connect it to your LabQuest by doing the following steps.
a.
b.
c.
At the top of the screen: Sensor  WDSS Setup
If it does not automatically scan, press Scan and select the name that is on the side of the WDSS
Select WDSS: Force
3.
Plug the Motion detector into the USB port of the LabQuest 2.
4.
Place the track on a level surface. Confirm that the track is level by placing the low-friction cart on the track and releasing it
from rest. It should not roll. If necessary, adjust the track.
5.
Attach the rubber band cord to the cart and then the cord to the string. Line up the Track and Force Sensor in a straight line
and adjust the track so that the cart can roll freely with the cord slack for most of the track length, but be stopped by the
cord before it reaches the end of the track.
6.
Zero the Force Sensor by doing the following steps.
a.
Sensor  Zero  Force Sensor
7.
On the graph screen change the top graph to Velocity by clicking on the y-axis and selecting velocity. Change the bottom
graph to a Force graph.
8.
Practice releasing the cart so it rolls toward the Motion Detector, bounces gently, and returns to your hand. The Force
Sensor must not shift and the cart must stay on the track. Arrange the cord and string so that when they are slack they do
not interfere with the cart motion. You may need to guide the string by hand, but be sure that you do not apply any force to
the cart or Force Sensor. Keep your hands away from between the cart and the Motion Detector.
9.
Click
to take data; roll 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. Roll the cart with a lower initial speed. If the
velocity graph has a flat area when it crosses the time-axis, the Motion Detector was too close and the run should be
repeated.
Once you have made a run with good position, velocity, and force graphs, analyze your data. To test the impulse-momentum
theorem, you need the velocity before and after the impulse.
10. Find the initial velocity of the cart by doing the following steps.
a.
b.
c.
Highlight the region the velocity graph when the cart was moving at a constant velocity before the impulse. This is
the flat section of the graph before the rubber band begins making a force on the cart.
At the top of the screen: Analyze  Statistics  Velocity
Read the MEAN velocity (it should be negative) and record the value for the initial velocity in your data table.
11. Find the final velocity of the cart by doing the following steps.
a.
b.
c.
Highlight the region the velocity graph when the cart was moving at a constant velocity after the impulse. This is
the flat section of the graph after the rubber band has gone slack again.
At the top of the screen: Analyze  Statistics  Velocity
Read the MEAN velocity (it should be positive) and record the value for the final velocity in your data table.
12. Now record the average force and the time duration of the impulse.
a.
b.
c.
d.
On the force vs. time graph, drag across the impulse, capturing the entire period when the force was non-zero.
Analyze  Statistics  Force
Read the MEAN force and record the value for the average force in your data table.
While the statistics box is open, scroll down to find the number of samples. To find the time interval for the
impulse take the number of samples and divide it by 50 (just trust the teacher on this one). Record the time value
for Duration of Impulse in your data table.
13. Perform a second trial by repeating Steps 10 – 12, record the information in your data table.
14. Change the elastic material attached to the cart by adding a second set of rubber bands.
15. Repeat Steps 8 – 13, record the information in your data table.