Question

Conceptual physics problems

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Answer 1

I need help with all five questions.

Answer 2

I feel that the correct answers for questions 1-3 are A, B, and B respectively. However, my professor can sometimes give tricky questions, so I'm not sure.

Answer 3

I'm clueless about 4.

Answer 4

Well, now that I think about it, is the answer C for 4?

Answer 5

I agree on 1 and 3.

I think that in 2, because each has an external torque due to frictional contact with the other that each experiences a change in angular momentum, therefore, the total angular momentum decreases.

For 4 I think that it is A because the angular momentum of the spinning disc should does not change the fact that it has mass and is attracted to the earth.

I'm not sure about 5.

Answer 6

For five it seems the answer is B. But I am also unsure.

Answer 7

Anyone else?

Answer 8

Sorry, I meant ANGULAR momentum.

Answer 9

Also, for 2, why do you think it says "the spheres interact only with themselves"? I know that if this were linear momentum, he answer would undoubtedly be B. But the question is asking for ANGULAR momentum. would this be enough to change the answer to C?

Answer 10

Well 1 is A.

2 is B because the TOTAL angular momentum is conserved if there is no external torques regardless of friction.

I would say 3 would be B.

4 is a little tougher, but the keyword is downward LINEAR acceleration so I would say it is A.

For 5, an object in static equilibrium must have no external or torques or forces acting upon it so it would be B.

Answer 11

However 4 is the only one I am a little Leary off.

Answer 12

I think that for 2 since there is a torque on each system that slows down each sphere that the total angular momentum changes. I think if they interact only with themselves means to consider them as a system. But there is energy lost, due to friction so the spheres will slow down, resulting a a change in angular momentum.

Answer 13

@Schrodingers_Cat, what if the friction causes both sphere's to stop spinning, wouldn't the angular momentum then be zero for each? And therefore the whole system would be zero as well?

Answer 14

Hmmm...I hadn't thought of that

Answer 15

Remember that even though friction is not conservative it is not an EXTERNAL force it is internal as such angular momentum at a point is always conserved if there is no external torque.

Answer 16

Hmm...
http://answers.yahoo.com/question/index?qid=20090330132756AAlEBsy
Could the answer to 4 be C?

Answer 17

Well, so far I'm feeling very good about 3 and 5, but I still have a lot of doubt about the other three.

Answer 18

I WILL give a medal to the best response!

Answer 19

1. A I=MR^2 approx. and masses for 1 are much farther from c.m. than for 2.

2 C. I liked B for awhile, but the friction argument swayed me, they will both slow down a bit.

3. B 4. Like ice skaters pulling in their arms, speeding up their spins, as angular momentum is conserved but moment of inertia is reduced.

4 C The energy from falling mgh is shared between kinetic energy 0.5mv^2 of the center of mass (c.m.) and rotational energy (0.5) I omega^2. This slows the vertical acceleration.

5. B In static equilibrium the linear and angular accelerations are zero, which is why we solve the problems by setting the forces equal to zero and the torques equal to zero.

Answer 20

@douglaswinslowcooper I gave you the medal because you were the one I felt gave the best answer for question 4. The question that still bothers me is 2.

I guess what is bothering me is that I am used to seeing conservation of momentum in linear terms: regardless of the internal forces acting within a system, as long as there are no external forces involved, momentum is conserved.

Is the the friction between the two spheres an exterior fore?