ANSWERS 333 PHYSICS – CIRCULAR AND ROATIONAL 2
1. A ladybug sits halfway between the axis and the edge of a phonograph record. What will
happen to its tangential speed if the RPM rate is doubled? At this doubled rate, what will
happen to its tangential speed if it crawls out to the edge?
A: tangential speed doubles. Assuming rate stays same, tangential speed will double as
radius increases.
2. A large wheel is coupled to a wheel with half the diameter as shown. How does the
rotational speed of the smaller wheel compare with that of the larger wheel? How do the
tangential speeds at the rims compare (assuming the belt doesn't slip)?
A: rotational speed of smaller wheel is twice that of larger wheel. Tangential speeds are
same since speed of belt is same everywhere.
3. An automobile speedometer is configured to read speed proportional to the rotational
speed of its wheels. If larger wheels, such as those of snow tires, are used, will the speedometer reading
be high or low—or no different?
A: For the same # of rotations, a larger wheel will move further down the road. Therefore, it covers more
ground in same time as ‘stock’ wheel, meaning car is going faster than reading indicates.
4. Harry and Sue cycle at the same speed. The tires on Harry's bike are larger diameter than those on
Sue's bike. Which wheels, if either, have the greater rotational speed?
A: Larger wheel is spinning slower, lower rotational speed.
5. A basketball player wishes to balance a ball on his fingertip. Will he be more successful with a spinning
ball or a stationary ball? What physical principle supports your answer?
A: Spinning ball has more stability because it has rotational inertia and doesn’t want to change axis of
rotation. Moving bike doesn’t fall like a stationary bike.
6. Compare the falling of two vertical meter sticks; one against a wall, and the other on a perfectly smooth
floor. How do the paths taken by their center of masses compare?
A: The Stick against the wall, CG moves outward in an arc towards floor. Stick that doesn’t have wall,
bottom will slide out and CG moves straight down.
7. If you walk along the top of a fence, why does holding your arms out help you to balance?
A: Increases rotational inertia since mass moves from rotation axis
8. The front wheels located far out in front of the racing vehicle help to keep the vehicle from
nosing upward when it accelerates. What physics concepts plays a role here?
A: dragster has very powerful engine that would cause it to pop a wheely about back
wheels. Extend front to move mass from back wheels to increase rotational inertia.
9. Which will have the greater acceleration rolling down an incline—a bowling ball or a
volleyball? Defend your answer. A: Bowling ball since overall mass is closer to center, less rotational
inertia and easy to get rolling
10. Which will roll down an incline faster, a can of water or a can of ice? Tricky one.
A: Water because it ‘slides down’ and isn’t made to roll. Sliding beats rolling.
11. Explain why a long pole is more beneficial to a tightrope walker if the pole droops.
A: lowers CG and makes more stable
12. Why must you bend forward when carrying a heavy load on your back?
A : To shift CG back over your feet
13. Why is it easier to carry the same amount of water in two buckets, one in each hand, than in a single
bucket? A: Keeps CG over feet
14 A: mass shifts out from cabinet and CG is not over base, it can topple.
15. A: lower truck since CG line won’t be over wheel base
16. A: you want to go tangent, away from circle, door presses you inward, giving appearance of outward
or centrifugal force.
17. A: you want to move tangent and fly off but wall is in your way. This makes you feel like you have
gravity. The pressing of your body. However, only force is center seeking.
18.A: Rate slows as mass gets further away
19. A: more mass away from rotation increases rotational resistance and we slow down, day gets longer.
Leaves fall, mass comes closer, rotation gets easier, days get shorter.
20. A: less mass at edge (equator) therefore, earth rotation is easier and speeds up. Day is shorter.