# Turntable Rotating on Shaft Problem

• BrainMan
In summary, the conversation discusses finding the acceleration and torque of a turntable, taking into account the moment of inertia of the shaft and the torque due to friction. The solution involves using the equations ωf = ωo + αt and ∑τ = Iα, and factoring in the moment of inertia of the shaft. After some discussion and adjustments, the correct answer of 0.52 N for the friction force is obtained by using the correct radius in the equation.
BrainMan

ωf = ωo = αt

∑τ = Iα

## The Attempt at a Solution

[/B]
1) Find the acceleration

ωf = ωo + αt

fo) / 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

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

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

BrainMan
BrainMan said:
τfriction = (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
gneill said:
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 = Itotalα

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

=> Ffriction = ∑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.

BrainMan said:
=> Ffriction = ∑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
TSny said:
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!

## 1. What causes a turntable to rotate on its shaft?

There are several possible causes for a turntable to rotate on its shaft, including misalignment between the turntable and the shaft, worn or damaged bearings, or an issue with the motor or power supply.

## 2. How can I fix a turntable that is rotating on its shaft?

The solution to this problem will depend on the specific cause. Some potential solutions include realigning the turntable and shaft, replacing worn or damaged bearings, or troubleshooting and repairing any issues with the motor or power supply.

## 3. Is it safe to use a turntable that is rotating on its shaft?

It is not recommended to use a turntable that is rotating on its shaft, as it can cause damage to the turntable and affect the quality of the sound produced. It is important to address the issue as soon as possible to prevent further damage.

## 4. How can I prevent a turntable from rotating on its shaft in the future?

Regular maintenance and proper handling of the turntable can help prevent this issue from occurring. This includes keeping the turntable clean and free of debris, avoiding excessive force or pressure when handling, and ensuring proper alignment when setting up the turntable.

## 5. Do I need to replace the entire turntable if it is rotating on its shaft?

Not necessarily. Depending on the cause of the problem, it may be possible to repair or replace only the affected components, such as bearings or the motor. However, if the issue is severe or the turntable is old and worn, it may be more cost-effective in the long run to replace the entire turntable.

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