Conservation of Momentum in Two Dimensions
Studio Physics I
WHEN OPENING THE FOLLOWING MOVIE,
CHOOSE TO LOCATE 2 OBJECTS.
For this activity you will need a movie called "collision.mov". You can get it off of the CD (go to
VideoPoint folder, then Movies, then look for it in the list). You can also transfer it from the
course webpage (Go to “class activities”, scroll down to the bottom of the page, RIGHT CLICK
on the movie, choose “save link as” and watch were the file gets saved to. Once you have the
movie file, open the Videopoint software, chose open movie and open "collision.mov" .
1. The first thing that must be done is to calibrate our measurement tool (which is the software).
There is a meter stick shown off to the right in the first frame of the movie. Use this as your
known length. Click on the ruler icon and follow the instructions that appear on the computer
screen. Do not change “scale origin” or “scale type”.
2. Now collect position data for each puck. Take your data carefully to get a good result the first
time around. To do this, enlarge the movie window so that it fills your entire screen. Then,
center the cursor over a point on the puck on the right. Click on the point you have chosen.
Now do the same for the puck on the left. Once you have clicked on the second puck, the
movie will advance to the next frame. Take data for all of the movie frames. You need to always
click on the pucks in the same order.
3. When you are done taking data, plot and sketch the x and y velocities of each puck.
4. Make a careful estimate of the average velocity of each puck in the x-direction BEFORE THE
COLLISION.
5. Make a careful estimate of the average velocity of each puck in the y-direction BEFORE THE
COLLISION.
6. How does one get the total velocity of each puck from the information that we have about the
velocities in the x-direction and y-direction? What is the total velocity of each puck before the
collision?
7. What is the momentum of the two-puck system in the x-direction before the collision? (Each
puck has a mass of 50.5 grams) What is the momentum of the two-puck system in the y-direction
before the collision? What is the total momentum of the two-puck system before the collision?
8. Draw freebody diagrams for each puck during the collision. Should momentum in the x
direction be conserved in the two-puck system during the collision? Why or Why not? How
about the y direction? Justify your answers in terms of the freebody diagrams that you have
drawn. How would your answers change if we used only one of the pucks as our system.
9. What is the velocity of each puck in the x-direction and in the y-direction AFTER THE
COLLISION? What is the momentum of the two puck system in the x and y directions after
 1999, 2000 K. Cummings
the collision? What is the total momentum of the two-puck system after the collision? Show
all your work in making this calculations.
10. Does your data indicate that momentum is conserved in this collision? Justify why you say
yes or no in terms of the values you calculated from the data.
As preparation for your homework, work on the following problems as a group.
11. A 50 gram puck slides at 3 m/s across a sheet of ice (ignore any small amount of friction)
completely horizontally and to the right. An identical puck moves straight downward at 5
m/s. The two pucks collide and bounce off one another. Following the collision, one puck is
moving rightward at 2 m/s in the direction 10o below straight rightward. Draw a diagram of
the situation before the collision. Draw a diagram of the situation after the collision. What
are the x and y components of the other puck’s speed after the collision. What direction is
this other puck moving?
12. Consider the system of objects shown below. Draw this diagram on your own paper and
label the axis to reflect your own choice of scale and origin. (That is, you pick the numbers
that you want to associate with the little vertical lines, but note that the lines are equally
spaced.) Calculate the exact x coordinate of the center of mass of this system according to
your choice of coordinate system. Mark the location of the center of mass with an X on your
diagram.
3.3m
m
13. Does the location of the X in your diagram make sense physically (like in terms of a
balancing point)? Justify (explain) why you say yes or no.
14. Would the location of your X (center of mass mark) move if you had chosen a different scale
and/or origin? Why or Why not?
15. Would the numerical value that you calculated for the location of the center of mass have
changed if you had chosen a different scale and/or origin? Explain your reasoning behind the
answer you give.
16. A 50 gram puck slides at 3 m/s across a sheet of ice (ignore any small amount of friction)
horizontally to the right. An identical puck moves straight downward at 5 m/s. The two
pucks collide and somehow stick together. Following the collision, the two pucks move as
one. What are the x and y velocities of the two pucks stuck together? What is the magnitude
and direction of the total velocity of the two pucks (once stuck together)?
 1999, 2000 K. Cummings