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. Open the VideoPoint software. Close the introduction box. Choose open movie. Go
to <Your Physics1 Data Folder> and open PASCO106. After loading and clicking
on “OK” to track one object, your computer screen should look like the image below.
#4
#2
44
#522
2
#3
#6
2. We must calibrate our measurement tool next. For this software, that means telling
the computer what distance on the computer screen corresponds to a known distance
that is shown in the movie. For example, you can use either of the two meter-sticks.
Before you do this, in order to get more accurate data, you may wish to make the
movie window larger and rearrange the other windows for convenient access. To
make the movie window larger, click and drag a corner. 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. (The object of “known length” in this case is a meter
stick, so you need to click on the two ends of one of the meter sticks). 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
Copyright©1999 Cummings; Rev. 2003 Bedrosian
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
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. Use the software to plot x vs. t. This is done by clicking on icon # 5 (shown in the
picture 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?
6. Use the software to plot y vs. t. Make a sketch of the graph. Is vy constant or
changing? How do you know? What is ay?
7. 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? Use the software to plot ay versus t
and sketch that graph on your paper. Does your graph support your answers to this
question and the question above?
8. As time goes on, how does the ay affect the velocity in the x-direction?
9. According to your data, what is the value of xo for the motion of this ball? (This is
the x position of the ball at t = 0.)
10. Use the software to plot the velocity of the ball in the x direction. What is vo,x? (This
is the velocity in the x direction at t = 0.)
11. Rewrite xf = xo + vo,xt + ½ axt2 , replacing variables xo , vo,x , and ax with the values
that you determined above in questions 5, 9, and 10.
12. What is yo, what is vo,y what is ay? (Make any plots you desire in order to determine
these values)
13. Rewrite yf = yo + vo,yt + 1/2ayt2, replacing variables yo , vo,y , and ay with the values
that you determined in 12 above.
14. Calculate the values of x and y that you would expect to measure 0.30 sec after the
ball leaves its initial location using the expressions you found in 12 and 13. Show all
your work. What do 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? (Hint: Easy answer.)
15. 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? Your measured values and the values
that you calculated in the step above should be within 10%.
Copyright©1999 Cummings; Rev. 2003 Bedrosian
EXERCISES: Projectile Motion
16. Pick the best answer to the following question and explain your reasoning.
A boy standing on flat ground threw a baseball straight up to a maximum height of 20
meters and let it drop to the ground. It was in the air a total of t1 seconds. He then picked
up the ball and threw it at an angle. It reached a maximum height of 20 meters and hit the
ground 50 meters away. On the second throw, it was in the air a total of t2 seconds.
Neglecting air resistance, which statement below is true?
A)
B)
C)
D)
t1 > t2 .
t1 = t2 .
t1 < t2 .
There is not enough information to determine whether A, B, or C is true.
A woman playing handball hit a hard rubber ball with her hand and gave it an initial
velocity of 20 m/s at an angle of 10° above horizontal. The ball hit a wall 10 m
(horizontally) away from the woman’s hand. Use g = 9.8 m/s2 and neglect air resistance.
Let x be the horizontal direction and y be the vertical direction (positive = up).
17. What is the x component of the initial velocity?
18. What is the y component of the initial velocity?
19. How long does it take for the ball to hit the wall? Explain your reasoning, don’t just
write down some numbers.
20. How high is the ball above or below the height of the hand when it hits the wall?
Explain your reasoning, don’t just write down some numbers.
21. When the ball hits the wall, is it still going up or is it on its way down? How do you
know?
Copyright©1999 Cummings; Rev. 2003 Bedrosian