Circular motion involving friction and tension

[fighter2_40]

Homework Statement

There is a block on a turntable. It is attached to the center of the table by a string with the length .4m. Also the static friction constant between the table and the block is 0.8 .
If the maximum tension the string can support is 300n how fast can the table spin without the string snapping?

Homework Equations

mass (velocity)^2 / radius = Centripetal Force
mass x 9.8 [gravity] x U [friction constant] = Force of Friction

The Attempt at a Solution

mv^2/.4 = 300 + .8mg

V = sqr(120/m +3.136)

Where do I go from here? Can I find a quantitative value for velocity if the mass of the block is not given?

Thanks! I would appreciate it if I can get an answer tonight. My impression is that this is due tomorrow.

 

[rl.bhat]

The block is accelerating towards the center of the turntable. So the frictional force on the turntable is away from the center. Reaction to this frictional force on the block is towards the center.
Now rewrite the equation.

 

[fighter2_40]

rl, I'm not sure I follow. I understand that the block is accelerating towards the center. Aren't i only concerned with the friction force on the block, which is towards the center (this is part of the forces necessary for allowing the block not to move to a farther orbit).

What I don't understand is why my equation mv^2/r = Ftension + Ffriction towards the center is incorrect. Further I don't know how to solve this without a given mass. Perhaps my teacher had forgotten to give the mass.

 

[rl.bhat]

You are right. Mass must be given to solve the problem.