Chapter 12.1-12.2 Questions
1. How does Newton’s first law of motion apply to rotating objects?
A rotating object tends to keep rotating about that axis. A non-rotating object tends
to remain non-rotating.
2. What is meant by rotational inertia?
The resistance of an object to changes in its rotational motion.
3. How does rotational inertia relate to rotational speed?
The greater an object’s rotational inertia, the more difficult it is to change the
rotational speed of the object.
4. What is required to change the rotational state of motion of an object?
A torque is required to change the rotational state of motion of an object.
5. What happens in the absence of a net torque?
In the absence of a net torque, a rotating top keeps rotating, while a non-rotating top
stays non-rotating.
6. What does rotational inertia depend on (2 things)?
Rotational inertia depends on mass and distribution of the mass.
7. How does the stick help the tightrope walker?
The greater the distance between an object’s mass concentration and the axis of
rotation, the greater the rotational inertia. By holding a long pole, the tightrope
walker increases his rotational inertia, allowing him to resist rotation.
8. How do baseball players make use of the rotational inertia of the baseball bat? Why?
Long baseball bat held near its thinner end has more rotational inertia, so it is harder
to bring the bat up to speed. Using a shorter bat, or choking up on a longer bat
reduces the rotational inertia, so the batter can bring it up to speed faster.
9. What does a short pendulum do as compared with a longer pendulum?
Short pendulum has less rotational inertia, so its frequency is higher (more swings in
the same amount of time).
10. How can you change the rotational inertia of your legs?
To reduce the rotational inertia of your legs, bend your knees.
11. What is the general formula for rotational inertia?
m = mass, r = radius. I = moment of inertia (rotational inertia). I = mr2.
12. Draw the 7 figures shown in 12.6 and list their formulae for rotational inertia.
13. How does rotational inertia affect how easily the rotational speed of an object changes?
The greater an object’s rotational inertia, the more difficult it is to change the
rotational speed of the object.
14. What are the 3 principal axes of rotation in the human body? Sketch them.
The 3 axes of rotation in the human body are the longitudinal axis, medial axis, and
transverse axis.
15. Describe the longitudinal axis.
The longitudinal axis is the vertical head-to-toe axis.
16. Compare the rotational inertia of the human body along the longitudinal axis.
Rotational inertia is the least about the longitudinal axis because most of the mass is
concentrated along this axis.
17. Describe the transverse axis.
The transverse axis is the axis you rotate around when you perform a somersault or
flip.
18. Compare the rotational inertia of the gymnast shown in figure 12.9 in each of the 4
positions.
a. Rotational inertia is least in the tuck position (a).
b. Rotational inertia is 1.5 times greater than the tuck position (b).
c. Rotational inertia is 3 times greater than the tuck position (c).
d. Rotational inertia is 5 times greater than in the tuck position (d).
19. How does a gymnast complete 2 or 3 somersaults before hitting the ground?
Rotational inertia is greater when the axis is through the hands. Rotational inertia is
up to 20 times greater when fully extended as opposed to being in the tuck position.
As the gymnast’s rotation transfers from one axis to another, the rate of rotation also
increases (up to 20 times faster). So, the gymnast is able to complete 2-3 somersaults
prior to hitting the ground.
20. Describe the medial axis. Give an example of rotation around the medial axis.
The medial axis is the front-to-back axis. The cartwheel is an example of rotation
around the medial axis.
21. What are the 3 principal axes of rotation in the human body?
The 3 principal axes of rotation in the human body are the longitudinal, transverse,
and medial axes.
22. Place the 3 principal axes of rotation in order from least rotational inertia to most
rotational inertia.
From least rotational inertia to most: longitudinal, transverse (when in the tuck
position and increasing as the position changes), medial. Rotational inertia can be
varied with different body configurations.