# Rotational Form of Newton's Second Law - Help

## Homework Statement

A turntable must spin at 33.3RPM (3.49 rev/s) to play an old fashioned vinyl record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 2 revolutions, starting from rest? The turntable is a uniform disk of diameter .305m and mass 0.22kg.

## Homework Equations

I = 0.5MR²
$\tau$ = $\alpha$I
$\alpha$ = (ωf-ωi)/t

## The Attempt at a Solution

I=(0.5)(0.22kg)(.1525²)=2.56x10^-3 kgm²

Redbelly98
Staff Emeritus
Homework Helper

## Homework Statement

A turntable must spin at 33.3RPM (3.49 rev/s)...
There's a little problem with your conversion of units for ω. Instead of "rev/s", it should be 3.49 ___/s (?)
...to play an old fashioned vinyl record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 2 revolutions, starting from rest? The turntable is a uniform disk of diameter .305m and mass 0.22kg.

## Homework Equations

I = 0.5MR²
$\tau$ = $\alpha$I
$\alpha$ = (ωf-ωi)/t

## The Attempt at a Solution

I=(0.5)(0.22kg)(.1525²)=2.56x10^-3 kgm²
Yup, that's I.

Your book should have even more relevant equations for rotational motion. You want one that involves θ, so that you can use the information that it takes 2 revolutions to get the turntable up to speed. You can check in your textbook for the full list of equations.

Starting from rest at a point O, let's call it, the motor supplies a torque so that by the second time we pass O, the angular speed is 3.49 rev/s. Based on this, you can use one of the kinematics equations (re-vamped into their respective rotational forms) and then incorporate the mass of the disc to find the torque.

There's a little problem with your conversion of units for ω. Instead of "rev/s", it should be 3.49 ___/s (?)

Yup, that's I.

Your book should have even more relevant equations for rotational motion. You want one that involves θ, so that you can use the information that it takes 2 revolutions to get the turntable up to speed. You can check in your textbook for the full list of equations.

Yes, it is 3.49 rad/s, sorry :P

Anyways, I found out the answer. My problem was that I didn't know theta was used as the 2 revolutions.

Redbelly98
Staff Emeritus