TBR Physics chapter 2 question 23

[mzblue]

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I can't understand the reasoning for this question. It's one of the free standing questions on the 25 questions review exam.

If a person starts at the rim of a spinning platform and is pushed radially toward the central axis by a moving exterior wall, then what happens to the normal force felt by that person due to the wall?
A. It remains constant
B. It decreases, since r decreases
C. It increases, since r decreases.
D It decreases, since angular speed decreases

If the person is being pushed toward the central axis? then r decreases. I'm assuming if F = mv^2/r, then the force should increase? The answer is B.
Somehow the whole thing is not clicking? it argues with angular velocity. First of all, does mcat even test angular velocity? i'm i wasting my time. I did this in physics one and it wasn't this complicated.
Thanks.

 

[Rabolisk]

There was a very similar thread earlier today. The key to note is that centripetal force is mv^2/r or mɷ^2*r, both of which are valid. You know that r decreases, but ɷ is constant, while v is not constant, which means that force decreases.

Remember that v is tangential speed, which is a function of the radius or circumference, whereas ɷ, angular speed is not affected in this situation. What really happens is that both v and r decrease at the same rate, which mathematically leads to mv^2/r DECREASING. Convince yourself that v decreases while ɷ does not.

 

[shffl]

Thanks

 

[mzblue]

Thanks. I did a search but the didn't see any thread. Just saw it but the title didn't draw me to it because the title stated content example and i thought it was one of the examples and not the exam question. Maybe it's a content example in the old book.

There was a very similar thread earlier today. The key to note is that centripetal force is mv^2/r or mɷ^2*r, both of which are valid. You know that r decreases, but ɷ is constant, while v is not constant, which means that force decreases.

Remember that v is tangential speed, which is a function of the radius or circumference, whereas ɷ, angular speed is not affected in this situation. What really happens is that both v and r decrease at the same rate, which mathematically leads to mv^2/r DECREASING. Convince yourself that v decreases while ɷ does not.

 

[stuw]

Kinda bringing this thread back from the dead, but could someone clear something up for me?

So is it safe to say that objects farther from the central axis that is undergoing uniform centripetal motion will have a GREATER tangential speed compared to one closer to the central axis? If so, why is this the case?

Also, regarding this question, if radius decreases, I can see it decreasing the tangential speed of the object, but how can we mathematically conclude and verify that this will lead to a decrease in the normal force?