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Flinthill84
Nov7-04, 06:45 PM
I have been working on this problem all weekend and I just can't get it!!

A small object is placed 10cm from the center of a phonograph turntable. It is observed to remain on the table when it rotates at 33 1/3 rev/min but slides off when it rotates at 45 rev/min. Between what limits must the coefficient of static friction between the object and the surface of the turntable lie? Calculate the value of Fc.

I believe I found the first part of the question by taking Us=v^2/g*r for each value that it rotates. I think that I need to use the formula Fc=m*v^2/r for the second part of the problem but I have no clue how to find the mass and have looked all through my notes and physics book. Please if you could help me I would greatly appreciate it.

Leong
Nov7-04, 07:18 PM
What is Fc ?

Flinthill84
Nov7-04, 07:22 PM
I believe it is the force

Phymath
Nov7-04, 07:26 PM
its centrip force

Flinthill84
Nov7-04, 07:26 PM
it is actually F sub c in the problem....centripetal force

Flinthill84
Nov7-04, 07:41 PM
yeah it is centrip force can anyone please help!!

Leong
Nov7-04, 07:59 PM
are you allowed to write Fc in term of m?

Flinthill84
Nov7-04, 08:06 PM
yeah I am pretty sure I am

Leong
Nov7-04, 08:12 PM
then just go ahead with it. give the range of Fc using the range of your coefficient of static frictional.

Flinthill84
Nov7-04, 08:16 PM
how do I go ahead with it?? I don't know the mass

Leong
Nov9-04, 08:00 PM
i thought you said that you were allowed to write Fc in term of m ?

Leo32
Nov10-04, 05:05 AM
Flinthill,

Just start working on the problem using a symbolical "m" for mass. If the figure is not in the original problem, this usually means it will cancel out somewhere.

Think of it as a nice aid to proove your solution: if the mass doesn't cancel out in your solution, you might be doing something wrong...

I usually only fill out the numbers once I have a general solution written out in symbols.

Greetz,
Leo