Studio Physics I
Introduction to two-dimensional Motion
In order to receive credit for a written answer, it must be expressed as a complete sentence.
1. Get a movie called "Projectile.mov” from your harddrive,the cd, or the web. It will be
located in the “videopoint” folder. Open the movie by double clicking on it. After
loading, your computer screen should look like the image below.
#4
verticle
#
2
#5
#3
#6
2. Click “OK” to track one object. Next, we must calibrate our measurement tool. For
this software, that means telling the computer what distance on the computer screen
corresponds to a shown in the movie that we know the value of. For example, you
can use either of the two meter-sticks shown in this movie and the picture above. To
calibrate the software, click on the ruler icon (#2 in the picture shown above) and
follow the instructions that appear on the computer screen. Do not change “scale
origin” or “scale type”.
3. Click on the icon #3 shown in the figure above. This will instruct the software to
leave a trail of markers at every location that you point the cursor to and click.
4. Center the cursor over the small white ball shown in the figure and click. The movie
will advance to the next frame and a small o should appear at the location of the
cursor at the time you clicked the mouse. Take the time to center over the ball VERY
carefully. It is easier to take good data the first time than to have to re-measure later.
Collect seven data points by leaving seven points on the screen. Note: When the
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cursor is in the movie window, it should look like the top icon (#4 in the figure
above). If the cursor does not look like this when you have it centered on the ball,
click on the top icon (#4) and the problem should be corrected.
5. In terms of x, y and t, what is the trail of circles a graph of?
6. Plot x vs. t This is done by clicking on icon # 5 above. Make a sketch of the graph. Is
the velocity in the x-direction (Vx) constant or changing? How do you know from the
graph? What is ax?
7. Plot y vs t
Make a sketch of the graph. Is Vy constant or changing? How do you
know? What is ay?
8. Consider an object that is shot through the air, like the ball in this video clip. How
does the acceleration due to gravity change for the object as it rises, reaches its
highest point and then starts to come back down? Plot ay versus t and sketch that
graph on your paper. Does your graph support your answer to this question?
9. From just your understanding of this situation (a ball flying through the air), make a
sketch of ay vs. t for the motion of the ball. You don’t need to take any additional
data to do this. Mark on your sketch the region which corresponds to the rise of the
ball, the point at which the ball reaches its highest point and the region which
corresponds to the ball falling back toward the earth.
10. As time goes on, how does the ay affect the velocity in the x-direction?
11. According to your data, what is the value of xo for the motion of this ball?
12. Plot the velocity of the ball in the x direction. What is vo,x?
13. Rewrite xf = xo + vo,xt + ½ axt2 , replacing variables with the values that you
determined above.
14. What is yo, what is vo,y what is ay? (Make any plots you desire in order to determine
these values)
15. Rewrite yf = yo + vo,yt + 1/2ayt2, replacing variables with the values that you
determined above.
16. Calculate the values of x and y that you would expect to measure 0.30 sec after ball
leaves its initial location. Show all your work. What is it that you need to know
about the motion in the y direction in order to determine the x position of the ball at
t=0.30 seconds?
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17. Return to the movie in VideoPoint and take the last few data points in the clip. What
values of x and y do you measure at 0.30 sec? If your measured values and the values
that you calculated in the step above are within 10%, you are done.
Copyright@1999 Cummings