Laboratory Activity 9: Impulse
and momentum
Group member names:
-Yen-Lin Lu
-Adam Critchelow
-Bryan Phillips
Objectives

Explore the nature of impulse and momentum.

Calculate impulse and momentum.

Develop the impulse-momentum theorem.
Equipment

Dynamics track.

USB hub

Force sensor with large
rubber stopper attachment.

Computer with DataStudio


Dynamics cart
Motion detector.


Cart mass
Two USB links.

Foam block
Activity One: Nature of collisions
Experiment 1: Before setting anything else up, take the cart with the spring
plunger extended and collide it several times with the wall or another immovable
object. Observe qualitatively what happens.
Question 1: How was the cart moving before the collision? What was the net
force on it before the collision? How was the cart moving after the collision?
What was the net force on it after the collision? How was the cart moving during
the collision? What was the net force on it during the collision? Is the force
constant, or does it seem to vary?
The car was moving steadily. The net force would be a friction force from the
surface it was moving on. It hits the object and stops and slowly starts moving in
the opposite direction. It has a net force in the negative direction from its original
motion. The cart hit the object and stopped its forward motion. The net force on
it would be a normal force from the object onto the cart, the force is constant
because the object doesn’t change.
To get a better picture of what is going on, we can use a force sensor to read the
force and a motion detector to measure the motion. We will also spread the
collision out more by making it with a more ‘springy’ surface.
Experiment 2: Connect the force sensor and motion detector to the computer,
and open the file ImpulseAndMomentum.ds from the web page in DataStudio.
Attach the force sensor to the magnetic end of the cart, and place the large
rubber stopper on it. Add a couple of cart masses. Set the motion detector at
one end of the track and hold the foam block at the other end so that the force
sensor will hit the middle and no other part of the cart will touch it (especially not
the rim). You will need to use a book, your hand, etc. to brace it While holding
the force sensor’s cord, start taking data, push the cart toward the foam, allow it
to collide and bounce back. Make sure to zero the sensor before taking data, to
keep hands out of the way of the motion detector, to not pull or push with the
force sensor cord, and that no part of the cart except the rubber stopper touches
the foam block. Take several runs until you get a good one, and paste your data
below.
Questi
on 2: Observe your graph. What happens with the velocity and force before the
collision? What happens to them during the collision? What happens to them
after the collision? Using the physics you already know, explain why they act the
way they do.
The velocity is a negative constant and force is zero. During the collision the
velocity goes to zero and then increases in the positive direction, and force
increases to a peak during the collision. Velocity stays positive and force drops
to zero. The collision causes the force between the objects and changes the
velocity by changing the direction of the cart.
In your class, the quantity of momentum has been discussed, where p = mv.
Calculate the momentum of the cart before and after the collision. You will need
to measure the mass of your cart and force sensor, and read the velocity from
your data. (Don’t forget units!)
mass of cart:
.858kg
Velocity
Momentum
Just before collision
-.63
-.54
Just after collision
.29
.25
Difference
.92
.79
A quantity called impulse has been defined for you in lecture or in your
textbook. It combines the applied force and the time interval over which it acts. In
one dimension, for a constant force F acting over a time interval t , as shown in
the graph below, the impulse is
Impulse = Ft
As you can see, a large force acting over a short time and a small force acting
over a long time can have the same impulse.
Note that F t is the area of the
rectangle; that is, the area under the
force vs. time curve. If the applied force
is not constant, then the impulse can
still be calculated as the area under the
force vs. time graph.
Question 3: Select the region of collision on the force graph. Use the Area
function on the statistics button to find the area under the force-time graph, and
record the value here. How does this number compare to the change in velocity
and the change in momentum calculated above? Which does the impulse
correspond to? (Think about the units, as well.)
.614Ns, Impulse is equal to the change in momentum, so impulse corresponds to
the momentum.
Summary
The following questions will help you get the main ideas out of this lab. You
should find these straightforward questions, but take the time to talk it over with
your team and write complete answers to these questions. You may find your
answers here to be the most useful part of this lab down the road.
Summary 1: Write down a definition of momentum and a definition of impulse.
Momentum is the mass * velocity of an object.
Impulse is the force * the change in time.
Summary 2: How is impulse related to momentum? How is it related to force?
How can it be found from a force vs. time graph?
The change in momentum is equal to the impulse. Force is a component of
impulse. If you multiply force over the change in time you get the impulse.
Summary 3: Describe the evidence and reasoning that allowed you to conclude
your previous statement.
We found that the change in the momentum using the lab we just did was equal
to the impulse of the object. We also used the equations p=mv and impulse =
F*change in t. Using those equations we found that the change in p to be equal
to the impulse.