Activity 13 – Introduction to General Rotational Motion
Studio Physics I
Part A – Introduction to Rotational Inertia (Observations)
1. Consider the system of two point particles shown. Each particle has a mass of 50 g
and each is the same distance (12 cm) from the center (which is marked with a box).
There is a rod connecting the masses, but it is so light that it can be ignored. Calculate
the object’s rotational inertia for a rotation about the center point.
2. The locations of the point masses are now changed. They are pushed toward the center
of the rod. See the diagram below. The masses are now each 2 cm from the center point.
What is the object’s rotational inertia about the center point now?
3. Start LoggerPro with the file “Rotation.xmbl”. You can get it from the course web
page. Go to the activity listing and go to Activity 13, LoggerPro File. You can also find
this file on the Studio Physics CD in the Physics 1 folder.
4. Adjust the masses on the rod so that they are as close as possible to the center of the
rod and tighten the screws to hold them . Click the Collect button in LoggerPro and give
the end of the rod a firm push. Note the maximum angular speed (rad/s) and observe the
time it takes for the spinning rod to slow down to half of its initial speed. Divide the
change in angular speed by time for an approximate value of angular acceleration.
Note: We are concerned only with angular speed, which is the absolute value of the
number reported by LoggerPro, which might be positive or negative.
5. Now adjust the masses on the rod so that they are as close as possible to the ends of
the rod and tighten the screws. Click the Collect button in LoggerPro and give the end of
the rod a firm push. Try to use approximately the same impulse of your push as you did
in step 4. Note the maximum angular speed and time to slow down to half that speed.
Analysis
6. Consider linear motion for the following question: Based on Newton’s Second Law
and the Impulse-Momentum Theorem, if two objects at rest are given the same impulse
from a push and friction is low, which object will have the higher speed after the push,
the one with greater mass or the one with lower mass? (Hint: easy answer.)
7. For rotational motion, rotational inertia plays the same role as mass plays for linear
motion. Is the maximum angular speed higher or lower in step 5 compared to step 4?
Explain your answer using the concept of rotational inertia and the answer to 6 above.
8. In rotational motion, torque () plays the same role as force in linear motion.
Assuming the friction torque in the bearings is approximately constant, and using  = I 
(the rotational equivalent of F = m a), explain the difference between the average
acceleration from 4 versus from 5 above.
 1999-2001 K. Cummings; Rev. 08-Feb-07 Bedrosian
Part B – Velocity and Acceleration in Rotations (Analysis)
The CD shown at the right is spinning counter-clockwise (as seen
from above). At a certain instant in time, two points on the disk
A
(A and B) are located as shown in Figure 1.
B
9. Copy this picture onto your paper and draw arrows at each of the
points (A and B) which indicate the direction of the linear velocity of
each point at this moment.
Figur e 1.
10. How does the time needed for point A to complete one revolution compare to the
time needed for point B to complete one revolution? How does the distance traveled by
point A (along a curved path) in completion of one revolution compare to the distance
traveled by point B in completion of one revolution?
11. Based on your answers to question 10, how do the linear speeds of points A and B
compare? Use the idea that average speed is a distance to time ratio.
12. Figure 2 shows the disk a short time after the moment pictured
in Figure 1. During this time, the disk has rotated through a half of
one revolution. For each point, draw an arrow indicating the
direction of the linear velocity at the point now.
13. How do the linear velocities of each point compare between
this later moment in time and the first moment in time, pictured in
Fig. 1? Discuss both the magnitude and the direction of the
velocity. Is the linear velocity of a given point (say point A) on the
disk changing with time? Why?
A
B
Figur e 2.
14. Suppose the disk shown in Fig. 1 completes one rotation in 0.1 seconds. What is the
change in angle  (measured in radians) for A? Is it the same for B? What is the time

rate of change of ? ( / t) This is the magnitude of the angular velocity vector,  .

15. Use the right-hand rule to determine the direction of  and record it on your paper.
Use terms like right, left, toward the top of the page, out of the page, etc. Do the

magnitude and/or direction of  depend on the point we pick on the disk? If the disk

was rotating clockwise, how would that change the direction of  ?
16. Suppose the disk was rotating counter-clockwise (as seen from above) and speeding


up. The time rate of change of  is the angular acceleration,  . What would be the

direction of  in that case? What if the disk was slowing down? (See lecture notes.)
17. Suppose someone looked at the disk in Fig. 1 from the bottom, not from the top
where you are looking. Would that person agree with you that the disk is rotating
counter-clockwise? Would that person agree with the magnitude and direction you

picked for  using the right-hand rule?

18. Given the answers to the above (9-17), why is  a useful quantity for describing the
rotational motion of an object?
(There is no Exercise for this activity.)
 1999-2001 K. Cummings; Rev. 08-Feb-07 Bedrosian