Studio Physics I
Introduction to Circular Motion
1. Open the VideoPoint software. Close the introduction box. Chose open movie. Go to <Your
Physics1 Data Folder> and open HRSY001. After loading and clicking on “OK” to track
one object, your computer screen should look like the image below.
#4
#2
#5
#3
#6
2. First we must calibrate the distance scale. (Before you do that, you might want to expand the
size of the movie window and move the other windows around for more convenient access.)
The diameter of the Ferris wheel (shown on the first frame) can be used as the known length.
It is 16.98 meters. (This is the diameter of the inside wheel, not the outside edge of the carts.)
Click on the ruler icon (#2 in the picture shown above) and follow the instructions that appear
on the computer screen. You will need to change the “known length” field to match the
diameter of the wheel (16.98). 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 a point on the wheel that you think you can keep track of. We suggest
that you choose a point in the lower left-hand portion of the picture. This is because these
points remain easy to see for many frames of the movie. They will not be obstructed by
anything as the frames advance. (Note that the Ferris Wheel spins in the clockwise direction
as we view it.) Click on the point on the wheel that you have chosen. The movie will
advance to the next frame and a small O should appear at the location you clicked on. The
spot on the wheel that you chose to keep track of should have rotated through some angle.
Click on the spot in its new location. Collect about 20 data points.
5. Based on the trail of circles left on the movie image, does the Ferris Wheel appear to be
rotating with a relatively constant speed? Explain your reasoning.
Copyright © 1999, 2000, 2001 Cummings; Rev. 2004 Bedrosian
6. Use the software to plot the velocity in the x-direction. When you do this, there will also be a
column added to the data table (#6 in the picture on the first page) that shows the values of
the velocity at the different moments in time. (You may have to scroll over to see it). Plot
the velocity in the y-direction. Sketch the plots on your activity sheet. Is the velocity in the
x-direction constant? Is the velocity in the y direction constant? Write down the range of
values that you find for each.
7. In addition to the x and y components of velocity that we looked at above, let’s consider the
magnitude of the velocity of the wheel, or speed. In general, how does one get the magnitude
of the velocity from Vx and Vy (the components)? (If you aren’t sure, check page 35 of the
book.) Calculate the magnitude of the velocity from the x and y components in your data
table for the first two times listed in the table. Show all your work on your activity write-up
8. Use the software to plot the magnitude of the velocity of the Ferris Wheel. This is done in
the same way as you have made other plots, except you must chose MAGNITUDE (not x or
y) from the pulldown menu where you would usually chose x or y. Then chose “velocity”
and click ok. Do the first couple of data points agree with what you calculated above? (They
should.) Based on the graph, what is the approximate value of the average magnitude of
velocity? What is the largest difference between the average and one of the magnitude values
in the data table? What is this difference as a percentage of the average? (This percent
difference is a crude indication of the uncertainty in your measurement.) Is the magnitude of
the velocity (speed) relatively constant?
9. What is the magnitude and direction of the centripetal acceleration of a passenger on this
Ferris Wheel at the highest point? What is the magnitude and direction of the centripetal
acceleration of a passenger on this Ferris Wheel at the lowest point?
10. What forces act on a person at the top of the Ferris wheel? (Note that the person is upside
down at the top. Hence, the normal force from the seat on the person points downward.) What
forces act on a person at the bottom of the Ferris Wheel? What are the directions of these
forces? Draw a free-body diagram for the passenger at the highest point. Draw a free-body
diagram for the passenger at the lowest point. Show the direction of acceleration off to the
side of each of the free-body diagrams. Have your TA (or professor) check your work on
this question before proceeding.
11. Translate each of the two free body diagrams into an algebraic expression based on Newton’s
2nd Law (F = ma). Use the 6-step process we discussed in class. What value will you use in
the equations above for acceleration? Solve for the normal force of the seat on a passenger in
terms of the other symbols. Make sure your signs (+/–) are consistent with the directions
of the forces and acceleration.
12. If we were to measure a person’s “apparent weight” on a Ferris Wheel (or in an elevator or
anywhere else), what we would actually be measuring is the normal force exerted on the
person. What is the apparent weight of a person with mass = 68 kg or weight = 68*9.8 = 666
N when he is at the highest point on the Ferris Wheel in the video? What is the apparent
weight of the person at the lowest point?
13. If you were riding this Ferris Wheel upside down at the top, why wouldn’t you fall out of the
seat since all of the forces on you are down?
Copyright © 1999, 2000, 2001 Cummings; Rev. 2004 Bedrosian
Exercise
A jet aircraft is flying in a vertical circle in a maneuver called a “loop-the-loop.” At the top of the
circle, the jet’s speed is 245 m/s and the center of the circle is 1225 m directly below the jet. The
jet is flying upside down. The ratio of the normal force (N) from the seat on the pilot to the
weight of the pilot at rest on the ground (W) is commonly called the “g-force” on the pilot, a
dimensionless number. What is the g-force or N  W for this pilot? Use g = 9.8 m/s2.
14. Use the six-step process we discussed in class to analyze this problem. First, identify the
forces on the pilot. (Hint: There are two.)
15. Choose a coordinate system. We recommend you make the direction of acceleration the
positive direction to avoid problems with – signs. (What is the direction of acceleration?)
16. Draw a free-body diagram of the pilot. (Hint: The pilot is upside down. What does that
imply about the normal force?)
17. Figure out whether the forces you identified are positive or negative in your coordinate
system.
18. Write Newton’s 2nd Law (F = ma). This should come directly from your diagram.
19. Calculate acceleration based on what we learned in class. (Class notes or book page 60.)
20. Solve for N  W using algebra. Why didn’t we need to know the pilot’s mass?
21. If you were the pilot of the jet at the top of the loop (upside down), what would it feel like?
Would it feel like you were falling out of your seat or would you feel pressed into your seat?
22. Suppose the jet does the loop a second time, only this time the jet is right-side up at the top of
the loop. (This maneuver is called an outside loop.) If you were the pilot, would it feel like
you were rising out of your seat against the straps or would you feel pressed into your seat?
Pilots call this “pulling negative g’s.” Explain what they mean.
23. One more maneuver to execute, and then we can finish the exercise and land our jet. This
time, we will fly right-side up, adjusting the engine thrust and the control surfaces so that all
forces on the jet cancel except the force of gravity. The jet will follow a parabolic trajectory
that we previously studied: projectile motion. What would it feel like to people riding inside
the jet? Why might it be useful to fly a jet for 30 seconds or more on such a trajectory?
Copyright © 1999, 2000, 2001 Cummings; Rev. 2004 Bedrosian