Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Impulse and Momentum & Conservation of Momentum
Objective: to investigate the impulse-momentum theorem and explore conservation of
momentum.
Report:
Use this document as the template for your report. Turn in one group report. It must be
word-processed or very legibly handwritten. All graphs must be integrated into the text
(not attached at the end).
Part A: Impulse-Momentum Theorem
Objective
In this part of the lab you will conduct an experiment to convince yourselves that the
impulse-momentum theorem does indeed work as it should. You will measure both the
impulse on an object (you) and its (your) change in momentum. You will then compare
these two quantities and see if they are indeed the same.
Background
The impulse-momentum theorem states that an object’s change in momentum is equal to
the impulse it experiences:
p  J
In one dimension this simplifies to p=J.
During the experiment you will jump from some known height onto a force plate and
measure the force that you experience from the force plate during landing.
Your motion will consist of two parts: free fall and landing.
You will
A1. use kinematics/dynamics to find your momentum gained during free fall.
A2. use the impulse-momentum theorem to predict the impulse needed to make you
stop (landing), and
A3. measure the impulse you experience during landing with the help of the force plate.
A4. Compare your expected and measured impulse and discuss your result.
Record all other data that you think is relevant (hint: the height of the chair, and the mass
of the jumper).
Important Change: instead of a person jumping we will use the data from a weight
dropped from a height of about 70 cm.
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Part A1: Find the momentum gained during free fall
Use the free fall portion of your experiment to find your momentum just before landing.
Describe what you do, what quantities you measure and how you take your
measurements. Include a picture, showing all relevant quantities, a FBD if needed, etc.
Note: Your system should be the person. Don’t forget that direction matters, i.e. you need
a coordinate system.
Person’s momentum just before
landing:
pi = ________________
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Part A2: Use the impulse-momentum theorem to predict the impulse needed to
make you stop (landing)
Consider drawing a momentum-bar
graph to support your reasoning:
Person’s momentum after landing:
pf = ________________
Person’s change in momentum during
landing:
p = _____________
Calculate the impulse exerted on the
person during landing:
Jcalc = _____________
Part A3: Measure the impulse you experience during landing
Draw a FBD of the person during his/her interaction with the force plate.
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
As you jump, make sure you land VERY softly. This is not required by the physics of the
experiment but appears to work better with our equipment (I suspect the force plate
cannot adequately record very sudden changes) and gives much better results.
Insert a plot of your force table data here (not attached to the end):
Write in your graph and mark the time intervals during which the person
 has not yet interacted with the force plate,
 is in the landing process,
 is just standing on the force plate after landing.
Did the force plate record the net force? Explain.
Use the recorded data to find the person’s weight. Explain how you find this info from
your graph.
Person’s Weight: _________
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Use LoggerPro to find the total impulse exerted on the person during the landing process.
Explain your reasoning/calculations.
Total MEASURED impulse exerted
on person during landing.
Jmeas = ______________
Part A4: Discussion and Conclusion
Discuss how well your calculated and measured values for J agree. Calculate a relative
error and discuss possible reasons for discrepancies.
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Part B: Conservation of Momentum
Objective
You will conduct a series of experiments that explore our first conservation law,
conservation of momentum, and when the condition for using it is met.
Background
Conservation of linear momentum can be expressed as:
p  p f  pi  0
Momentum is conserved only if the net force on the system is zero, or Fnet  0 .
If you want to properly apply this conservation law it is crucial that you choose your
system such that it has no net force acting on it during the collision/explosion/interaction
that you are investigating. If there is a non-zero net force on your system there will be an
impulse and the momentum-impulse theorem applies.
Notes on today’s LoggerPro setup
You will be using two carts each of which has its own motion detector. Usually, the
motion detector is set up to measure position as positive away from the detector. For your
setup one motion detector works as usual (the one on the right), the other one is reversed
and the zero point is set at the right detector. This means that both detectors will measure
on the same x-axis! Ask your instructor if you have questions.
In all experiments LoggerPro is set up to display both the x-t and v-t graphs for both
carts.
Notation
Cart on the left: cart 1
Cart on the right: cart 2
m1
mass of cart 1
m2
mass of cart 2
v1
v1
initial velocity of cart 1
initial velocity of cart 2
p1
p1
initial momentum of cart 1
v2
v2
p2
p2
final velocity of cart 1
final momentum of cart 1
final velocity of cart 2
initial momentum of cart 2
final momentum of cart 2
Before you do any of the activities make a prediction of what you expect.
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Activity 1: (m1 = m2, Cart 1 is pushed against cart 2, cart 2 is stationary, magnetic
carts)
Start with cart 1 on the left cart 2 on the right. Give cart 1 a push towards cart 2 (which is
initially stationary). Predict x-t and v-t graphs, then do the experiment.
Paste a graph of x-t and v-t here:
Fill out the table below (use your LoggerPro data as necessary):
m1
v1
m2
v2
v1
v2
Questions:
1.1 Choose cart 2 to be your system.
Draw a FBD for your system during the collision.
Should momentum be conserved for your system? Why or why not?
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Calculate the momentum of your system before and after the collision. Was momentum
conserved?
1.2 Choose both carts to be your system.
Draw a FBD for your system during the collision.
Should momentum be conserved for your system? Why or why not?
Calculate the momentum of your system before and after the collision. Was momentum
conserved?
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Activity 2: (m1 no extra weight, m2 with one extra weight, cart 2 stationary, cart 1 is
pushed against cart 2, magnetic carts)
Put one extra weight (the black ‘sticks’) on cart 2.
Let cart 1 (on the left) be stationary and give cart 2 (on the right) a push towards cart 1.
Predict x-t and v-t graphs. How will cart 2 move after the collision? Conduct the
experiment.
Paste a graph of x-t and v-t here.
Fill out the table below (use your LoggerPro data as necessary):
m1
v1
m2
v2
v1
v2
Calculate the momentum of your system before and after the collision. Was momentum
conserved?
Lab Activity:
Momentum/Impulse and Conservation of Momentum
Phys 221
Activity 3: (m1 no extra weight, m2 with one extra weight, both carts pushed towards
each other, magnetic carts)
Put one extra weight (the black ‘sticks’) on cart 2.
This time give both carts a push towards each other. Predict how both carts will move
after the collision. Conduct the experiment.
Paste a graph of x-t and v-t here.
Fill out the table below (use your LoggerPro data as necessary):
m1
v1
m2
v2
v1
v2
Calculate the momentum of your system before and after the collision. Was momentum
conserved?