ICP/Physics 8
NAME
Collisions lab, v. 1.0
______________________________
Date _______________
TA _______________
p. 1
PARTNER'S NAME ___________________
Lab Day/Time _________________
LAB 4: COLLISIONS
Introduction
This lab investigates collisions, both elastic and inelastic. By exploring these different kinds
of collisions in detail, you will develop a better understanding of what is, and is not, conserved in
various types of interactions. This will in turn enable you to avoid some common mistakes in analyzing
scenarios that involve both energy and momentum considerations.
Equipment
Your TA will show how to use the photogate timers, and the plunger on the launch cart.
I. Launch velocity
Before running the collision experiments, we must determine the initial launch speed of t h e
launch cart—its speed right after it pushes off the end of the track.
♦
Make sure the two photogates are set up near the end of the track, between 5 and 10 cm apart.
♦
Measure the distance, ∆x, between the two photogates. You’ll get the most accurate reading
by measuring from the edge, not the center, of each photogate. For instance, you can measure
from the left edge of the start gate to the left edge of the stop gate.
♦
Launch the cart, using the plunger. Make sure the compressed plunger is touching the wall.
Practice this a few times until you can consistently launch the cart smoothly.
ICP/Physics 8
♦
Collisions lab, v. 1.0
p. 2
After these practice runs, measure the launch velocity at least four times using t h e
photogates, and find the average. (If one of these measurement results is clearly “off” from
the others, throw it out.) You’ll use this average as your “v0 ” for the rest of the lab.
Data and calculations for v0 :
1.
Of the measured launch velocities that you used in your calculation, which is farthest off from
the average? By what percentage does it differ? (This is one quick measure of the spread in your
data.)
ICP/Physics 8
Collisions lab, v. 1.0
p. 3
II. Completely inelastic (Velcro) collision
In this experiment, the launch cart will collide with a stationary second cart. You will orient
the carts so that their Velcro ends collide, which will make them stick together—a collision known as
completely inelastic. The stuck-together carts will then pass through the photogates, allowing you to
measure the post-collision velocity, vf. Don’t do the experiment until making predictions!
2.
(Prediction) Using any technique you’d like, solve for v f in terms of v 0 . Is v f half of v 0 , or three
quarters of v0 , or what? Both carts have the same mass. That m should cancel out of your answer.
♦
Making sure the Velcro end of the second cart faces the launch cart, do the experiment. Then
use your data to find vf in terms of v0 . (Run the experiment several times to get a solid result.)
Is vf (approximately) half of v 0 , three quarters of v0 , or what?
Data, calculations, and results:
ICP/Physics 8
3.
4.
5.
Collisions lab, v. 1.0
p. 4
Was kinetic energy conserved during the collision? Show your work to support your conclusion.
Reconcile energy conservation with the results of this experiment.
If you didn’t use momentum conservation in question 2, use it now to predict v f in terms of v 0 . Does
this prediction agree with the experimental result?
ICP/Physics 8
Collisions lab, v. 1.0
p. 5
III. Elastic (perfectly bouncy) collision
In this collision you will orient the carts so that the magnetic ends face each other. As a result,
the carts bounce off each other perfectly, instead of sticking together.
6.
Prediction: What will be the post-collision speed of each cart? Show your reasoning.
♦
Now run the experiment.
Data, calculations, and results:
ICP/Physics 8
Collisions lab, v. 1.0
p. 6
7.
( a ) Was kinetic energy conserved in this collision (to within experimental error)?
(b) Was momentum conserved?
8.
From the results of all these experiments, formulate general rules about collisions that specify
when momentum is conserved, and when kinetic energy is conserved.
IV. Carts with different masses
In the following experiments, the carts have different masses.
9.
Prediction: Suppose you double the mass of the launch cart, but continue to launch it using t h e
same plunger setting as before. Let v 1 denote the cart’s speed immediately after it launches.
Using whatever technique you’d like, calculate the new launch velocity, v1 , in terms of the “old”
launch velocity, v0 .
Is v1 equal to v0 ? Half of v 0 ? If you can’t figure it out, make an educated guess and explain your
rough reasoning.
ICP/Physics 8
♦
Collisions lab, v. 1.0
p. 7
Test your prediction. The metal bars have the same mass as a cart. So, placing a bar on top of
the cart doubles its mass. If the prediction was wrong, try to explain why.
Data, calculations, results:
If they disagreed: explanation for why , and correction
ICP/Physics 8
Collisions lab, v. 1.0
p. 8
10.
Prediction: If the double-mass launch cart (2m ) undergoes a completely inelastic (Velcro)
collision with the stationary second cart, what will be the final speed of the two carts after they stick
together? The second cart has the same mass as it did before (m).
♦
Test your prediction, and explain what’s going on if your prediction was wrong.
Data, calculations, results:
ICP/Physics 8
11.
Collisions lab, v. 1.0
p. 9
Now let’s think about the case where the double-mass launch cart undergoes a perfectly elastic
collision with the second cart.
( a ) (Prediction) After the collision, does the launch cart move forward, move backward, or come
to rest? Answer intuitively, with no calculations, and briefly justify your response.
(b)* Calculate the post-collision speed of the second cart, in terms of v 1 . (At least set up t h e
relevant, correct equation(s) to solve for this speed. If you lack time to finish t h e
calculation, do the experiment now, and then finish the calculation for yourself later.)
ICP/Physics 8
♦
Collisions lab, v. 1.0
Test your prediction about the post-collision speed of the second cart.
Data, calculations, results:
p. 10
ICP/Physics 8
Lab 4, Collisions – Extra questions
LAB 4 (COLLISIONS): Extra questions
Note: These extra questions, intended for people who finish the lab early, give you practice applying
the same concepts to harder situations. These questions are strictly for your benefit; they will not
count as part of your lab grade, and they do not cover new material. But they are designed to give
you practice on exam-level questions.
1.
If the launch cart is much heavier than the stationary second cart, then what is the postcollision speed of the second cart if. . .
( a ) The collision is inelastic (Velcro)? Explain both intuitively and mathematically.
(b)*
2.
Prediction: In an elastic collision, what happens if the initially-motionless second cart is much
much heavier than the launch cart? Figure it out both intuitively and mathematically.
♦
3
The collision is elastic (perfectly bouncy)? Again, figure it out both intuitively and
mathematically. Hint: Analyze the collision from the center-of-mass reference frame,
which in this case is essentially the same as the reference frame that rides along with t h e
launch cart. Please answer on back.
Try it, by piling several metal bars onto the second cart.
When you throw a ball of clay against a wall, the clay starts off with lots of momentum, and ends
up with no momentum. So, the initial momentum appears to be greater than the final momentum.
Is momentum conservation violated? Explain.
4.** Find a second launch cart. Using the same plunger setting, launch the two carts towards each
other from opposite ends of the track. Come up with a way to figure out what percentage of a
cart’s kinetic energy is lost (dissipated into heat, etc.) each time the cart collides with the wall.