Solving Static Friction & Force: Limits & Calculation | Turntable Problem

[Flinthill84]

problem with static friction and force

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]

What is Fc ?

 

[Flinthill84]

I believe it is the force

 

[Phymath]

its centrip force

 

[Flinthill84]

it is actually F sub c in the problem...centripetal force

 

[Flinthill84]

yeah it is centrip force can anyone please help!

 

[Leong]

are you allowed to write Fc in term of m?

 

[Flinthill84]

yeah I am pretty sure I am

 

[Leong]

then just go ahead with it. give the range of Fc using the range of your coefficient of static frictional.

 

[Flinthill84]

how do I go ahead with it?? I don't know the mass

 

[Leong]

i thought you said that you were allowed to write Fc in term of m ?

 

[Leo32]

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