Solving Rotating Heads Assessments: What's Correct?

[PhysicsNovice]

Hello Doc Al and others. I am progressing along with my on-line basic physics course. I completed a new module and have some assessment questions to answer. Please review and let me know the ones that are correct. Those that are wrong just a little clue as to where my thinking process went a stray. I will add my thought process to the ones that I missed telling how I came up with the answer that I did. Thanks for taking the time to assist me on some basic questions.

1. What is the rotational speed of the minute hand on a clock?
a. 0.000278 rev/s*
b. 0.00175 rev/s
c. 0.0167 rev/s
d. 0.105 rev/s

2. A child with a mass of 25 kg is riding on a merry-go-round. If the child has a speed of 3 m/s and is located 2 m from the center of the merry-go-round, what is the child's angular momentum?
a. 50 kg·m2/s
b. 75 kg·m2/s*
c. 150 kg·m2/s
d. 300 kg·m2/s

3. Newton's first law for rotational motion states that an object will maintain its state of rotational motion unless acted on by an unbalanced (or net):
a. force
b. velocity
c. inertia
d. torque*

4. A child with a mass of 20 kg sits at a distance of 2.0 m from the pivot point of a seesaw. Where should a 16-kg child sit to balance the seesaw?
a. 1.6 m
b. 2.0 m
c. 2.5 m*
d. 3.2 m

5. A solid sphere and a solid cylinder are made of the same material. If they have the same mass and radius, which one has the larger rotational inertia about its center?
a. the sphere
b. the cylinder
c. both the same*
d. There is not enough information to say

6. A fully-loaded trailer truck is less stable than a race car because the truck:
a. is more massive
b. has a higher center of mass*
c. has bigger wheels
d. weighs more



7. If you face a wall with your toes against the baseboard, you find that you cannot stand up on your toes because:
a. you cannot generate the necessary torque
b. your rotational inertia is too large
c. your center of mass is too high
d. your center of mass cannot move forward over your toes*

8. An astronaut "floating" in Skylab has an initial rotational motion but no initial translational motion relative to Skylab. She continues to rotate because she:
a. experiences no net force*
b. experiences a net force that acts through her center of mass
c. is weightless
d. experiences a torque due to the force of gravity

9. A spinning gyroscope points directly at the North Star when it is located at the North Pole. If it is transported to the South Pole without exerting any torques on it, which way will it point?
a. up
b. down*
c. horizontally

10. Assume that Gerry sits on a freely rotating stool holding a bicycle wheel with its axle vertical so that it rotates in a clockwise direction when viewed from above. If Gerry turns the wheel over, he will:
a. not rotate because the system of wheel and Gerry is closed or isolated
b. not rotate because the two torques cancel
c. rotate clockwise because angular momentum is conserved*
d. rotate counterclockwise because angular momentum is conserved

 

[Gokul43201]

What are the formulas involved in Q2 and Q5 ?

And Q8 has no correct choice among those listed, but if I had to pick one, I'd pick the same one you did.

 

[BobG]

Noticed the same thing Gokul did. There's a difference between linear momentum (the equation you used in 2) and angular momentum and difference between mass and moment of inertia (rotational inertia in your case).

 

[Doc Al]

I'll add a few comments to what Gokul43201 and BobG said:

5. A solid sphere and a solid cylinder are made of the same material. If they have the same mass and radius, which one has the larger rotational inertia about its center?
a. the sphere
b. the cylinder
c. both the same*
d. There is not enough information to say

You should be able to figure this one out without needing any fancy formulas. As long as you realize that when mass is arranged to be closer to the axis the rotational inertia decreases... and vice versa. So imagine a ball of clay of a certain radius. Now squash it into a cylinder shape--a "hockey puck" with the same radius. Do you end up pushing clay closer to or further from the axis?

8. An astronaut "floating" in Skylab has an initial rotational motion but no initial translational motion relative to Skylab. She continues to rotate because she:
a. experiences no net force*
b. experiences a net force that acts through her center of mass
c. is weightless
d. experiences a torque due to the force of gravity

Since you are learning about rotational motion you should realize that "no net force" does not insure a constant rotation, since you can certainly exert a torque and still have a zero net force. Furthermore, you should realize that Skylab and its contents certainly do experience a net force: gravity is what keeps them in orbit. The better answer is that gravity exerts no torque on the astronaut since it effectively acts through her center of mass.