# Homework Help: F=ma 2011 number 24

1. Jan 26, 2014

### licantroleon

1. The problem statement, all variables and given/known data

A turntable is supported on a Teﬂon ring of inner radius R and outer radius R+δ (δ  R), as shown in the diagram.
To rotate the turntable at a constant rate, power must be supplied to overcome friction. The manufacturer of the
turntable wishes to reduce the power required without changing the rotation rate, the weight of the turntable, or
the coeﬃcient of friction of the Teﬂon surface. Engineers propose two solutions: increasing the width of the bearing
(increasing δ), or increasing the radius (increasing R). What are the eﬀects of these proposed changes?

(A) Increasing δ has no signiﬁcant eﬀect on the required power; increasing R increases the required power.
(B) Increasing δ has no signiﬁcant eﬀect on the required power; increasing R decreases the required power.
(C) Increasing δ increases the required power; increasing R has no signiﬁcant eﬀect on the required power.
(D) Increasing δ decreases the required power; increasing R has no signiﬁcant eﬀect on the required power.
(E) Neither change has a signiﬁcant eﬀect on the required power.

2. Relevant equations
P=Fv=Tw

3. The attempt at a solution
I have no idea how to start this.... Also I don't understand the situation. Is the turntable like a box on top of the ring? Or do they mean they have a vinyl disc on top of the ring? I have no idea what they're talking about.

2. Jan 26, 2014

### Simon Bridge

What difference does it make if the turntable is a vinyl disk or some sort of box?
What matters is how the friction with the teflon ring depends on the dimensions of the ring.

Anyway - the diagram should give you a hint.

3. Jan 26, 2014

### licantroleon

I have no idea where the friction force would be located... Would it be concentrated at the middle of the ring width?

4. Jan 26, 2014