# Angular Momentum of a sanding disk

1. Nov 15, 2007

### Destrio

A sanding disk with rotational inertia 1.22x10^-3 kgm^2 is attached to an electric drill whose motor delivers a torque of 15.8 Nm
a) find angular momentum
b) find angular speed of the disk 33.0ms after the motor is turned on.

L = Iω
τ = Iα

we can find angular acceleration with what we are given α = τ/I
but I'm completely stumped on how to find L without having time.
any hints?

thanks

2. Nov 15, 2007

### Staff: Mentor

Here's a hint: You are given the time!

3. Nov 15, 2007

### Ojizo88

lol man I stared at that over and over again and thought, "I must be misreading this...wheres the trick?"

But seriously, yes you are given the time. its "33.0ms" part

4. Nov 15, 2007

### Destrio

Haha, thanks
I realized from that that I can use the time to solve the first part of my problem.

L = Iω

L =τω/α
ω = αt
L =τt

that works

τt = Iω
ω = τt/I
which gives me 427m/s
but i need the answer in rev/min
im okay with seconds to minutes, but how do i convert meters to revolutions?
2pi?

5. Nov 15, 2007

### Ojizo88

If you can find its angular acceleration, then ( just like linear kinematics... ) its angular velocity is equal to its angular acceleration multiplied by the duration of the acceleration.

6. Nov 15, 2007

### Staff: Mentor

The units for ω are radians/sec, not m/s. One revolution equals $2 \pi$ radians.

7. Nov 15, 2007

### Destrio

woo i got it!

thanks