# Rotational Motion of a spun disk

1. Nov 30, 2007

### jeeves_17

A 200 g, 20-cm-diameter plastic disk is spun on an axle through its
center by an electric motor. What torque must the motor supply to take
the disk from 0 to 1800 rpm in 4.0s? (Given: I = ½ MR2)

Relevant equations
I = 1/2 MR^2
T = Ia

The attempt at a solution
v=0.02m * 1800 rev/min / 4 sec * 60 sec / 1 min * 2 Pi / 1 Rev
v= 10 800 m/s

a = how do i get this?

I = 1/2 (0.2kg)(0.1)^2
I = 0.001

T = Ia
T = (0.001)a

is this correct?

2. Nov 30, 2007

### Staff: Mentor

What you need to compute is the angular acceleration ($\alpha$). Just like linear acceleration is defined as change in velocity over time, angular acceleration is change in angular velocity over time. What's the change in angular velocity? (Convert rpm to rad/s.)

3. Nov 30, 2007

### rl.bhat

Use kinematics w = wo + alpha*t. Here wo = zero and w = 2*pi*1800/60. From these values find alpha and then torque.