Simple Conceptual Problem

Question

wach · Answer

I entered that b and c were constant but that is wrong. Because it's a frictionless system is there anything lost?

wach · Answer

Mostly this concerns whether v or velocity is conserved, right? I mean if you break down the formulas.

anonymous · Answer

the torque is constant, being m g times the radius of the wheel, which is where the string tugs on the wheel. Third choice neglects KE of the falling mass. Torque produces increase in angular momentum, so it cannot be constant.

wach · Answer

The sum of the angular momentum is conserved, correct?

wach · Answer

Oh I see. Because of the change in r / f which is related.

wach · Answer

I entered a only and it's incorrect also. Since the angular momentum is proportional to m v and v seems to change as the mass changes direction, is it possible that the answer is 'none'?

wach · Answer

@douglaswinslowcooper  ?

anonymous · Answer

I did not like A because it refers to the angular momentum with respect to the wheels center, and also because we know the external force is not zero, but gravity, so angular momentum should change. Torque provided by the mass tugging on the string increases the angular momentum, caused by the external force of gravity.  Momentum and angular momentum are each conserved only when there are no external forces. Gravity is such a force. I still like B only.

wach · Answer

Okay. I'm going to ask my prof. just to be safe. Thanks for your time.

anonymous · Answer

You are welcome.

vincent-lyon.fr · Answer

I would say, only B is correct.

A is not correct because there is clearly an increase in angular momentum from 0 to non zero as the earth pulls on the system.

B is correct because the acceleration is constant, so the force of the string on the pulley is constant and its momentum too. BUT, unlike douglaswinslowcooper said above, that moment is not mgR, since the mass is accelerating, the tension in the string is not equal to mg.

C would be ok if the KE of the mass had been mentioned. But as it is not mentioned, C is incorrect.

anonymous · Answer

Good point about the force and torque on the wheel. I was wrong. They would be  mg and mg R if the wheel and the mass were stationery. With no friction and inertia of the wheel, the weight would simply accelerate at g, no tension on the rope. In reality, there will be friction and inertia, so some tension on the rope, putting some torque and changing the angular momentum over time.  Sorry to have been partially wrong.

wach · Answer

Thanks for the greater insight. I'm going to see if I can find more information on this because Physics Quest is worth a huge percentage of my grade.