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. The first thing that must be done is to calibrate our measurement tool. (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. 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.
Copyright © 1999, 2000, 2001 Cummings; Rev. 2003 Bedrosian
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.
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)? 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 second 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?
Copyright © 1999, 2000, 2001 Cummings; Rev. 2003 Bedrosian