TBR physics, a few questions..

[anbuitachi]

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Hi this is from the Torque chapter questions from berkeley review.. help appreciated.

Students decides to study physical variable that affect torque, using apparatus (below) that has weighted bar connected to spindle. Spindle turns when mass attached to cord that is allowed to freely fall.

Attached to spindle is light weight nylon cord ending in a loop from which weights of varying mass can be suspended. The spindle is mounted on shaft that has a perpendicular bar on top. Objects may be attached w/ screws at any point along teh bar. Students compare rate of rotation of the upper cross bar with the masses of objects affixed to it and w/ radius of spindle. mass of nylon cable is negligible and system is not frictionless.

1) In experiment after weight has reached lowest point and nylon cord is completely extended, what will be observed?
a) weight will remain stationary at lowest point
b) weight will rise back up to original position as spindle continues to turn in the same direction
c) weight rise back up to position slightly lower than original, as spindle continues to turn in same direction
d) weight will rise to position slightly higher than original, spindle continues to turn in same direction.


Can someone explain to me how this is done please? thanks

 

[RogueUnicorn]

i think the answer is C, and it's not so much a torque question as it is a question forcing you to think about what happens to the total mechanical energy of a system when friction is involved, which is to say, some of it is lost, meaning the the weight cannot regain its original height. this is much in the same way as a pendulum eventually coming to a stop in real life.

 

[loveoforganic]

Agree with bleargh.

 

[BerkReviewTeach]

i think the answer is C, and it's not so much a torque question as it is a question forcing you to think about what happens to the total mechanical energy of a system when friction is involved, which is to say, some of it is lost, meaning the the weight cannot regain its original height. this is much in the same way as a pendulum eventually coming to a stop in real life.

Bleargh is spot on.

I might add that if you are having a hard time with this example, think about how a yoyo works, although it's not the same thing it is based on a similar principle. The yoyo drops and creates torque which spins the plastic wheel of the yoyo. It spins fastest at its lowest point and then continues to spin as the yoyo string recoils around the plastic wheel as the yoyo rises back up. You have to tug slightly to overcome the loss of energy due to friction, or else the yoyo won't recoil all the way.

 

[Geekchick921]

Agree with bleargh.

I can't see the picture at work, but from what I got from the explanation of the apparatus and the question, I do, too.

 

[badmintondr]

Sorry for the severity of my previous post. Would someone mind clarifying the problem? I don't seem to understand how the rotational forces would keep the spindle up and how it wouldnt constantly bounce up and down from running out of nylon fiber. Thanks