Turntable Rotating on Shaft Problem

[BrainMan]

Homework Statement

Homework Equations

ω_f = ω_o = αt

∑τ = Iα

The Attempt at a Solution

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1) Find the acceleration

ω_f = ω_o + αt

(ω_f-ω_o) / t = α = -0.231 rad/sec

2) Find ∑τ

∑τ =Iα

I assumed that the only torque acting on the turntable was the torque due to friction

τ_friction = (1/2)MR²(-0.231) = -.00312 N*m

 

[TSny]

Looks OK so far. How do you get the friction force from the torque?

 

[gneill]

τ_friction = (1/2)MR²(-0.231) = -.00312 N*m

I didn't see a justification for ignoring the moment of inertia of the shaft (I'm assuming that the M and R above are for the disk).

 

[BrainMan]

I didn't see a justification for ignoring the moment of inertia of the shaft (I'm assuming that the M and R above are for the disk).

So if I factor in the moment of inertia of the shaft I get the ∑τ = I_totalα

=> [(1/2)MR² + (1/2)mr² ](-0.231) = -0.00312 N*m

=> F_friction = ∑τ / (R+r) = -0.020 N

The correct answer should be 0.52 N so I'm not sure how I'm so far off.

 

[TSny]

=> F_friction = ∑τ / (R+r) = -0.020 N

The correct answer should be 0.52 N so I'm not sure how I'm so far off.

Why use R + r ? Note that the brake pads are applied to the shaft.

 

[BrainMan]

Why use R + r ? Note that the brake pads are applied to the shaft.

Okay if I use r instead of R + r I get the right answer. Thanks!