# Homework Help: (P.20) Angular Acceleration

1. Jun 9, 2014

### gcombina

A grindstone,initially at rest,is given a constant angular acceleration so that it makes 20.0 rev?
A grindstone,initially at rest,is given a constant angular acceleration so that it makes 20.0 rev in the first 8.00 s. What is its angular acceleration

My attempt:

α= Δω/ΔT

I converted 20 rev/8s to radians and I got 15.7 rad/s, then I plugged in the numbers into the equation above ^

What am I doing wrong?

2. Jun 9, 2014

### CAF123

Your work assumes that the stone moves at constant angular velocity.

3. Jun 9, 2014

### verty

Draw a graph of radians versus time, that should help.

4. Jun 9, 2014

### Nathanael

You answered the question as if it said:
"A grindstone, initially at rest, is given a constant angular acceleration so that it is going 20.0 rev/s after the first 8.00 s"

See the difference?

5. Jun 11, 2014

### gcombina

so i guess I should figure out the angular velocity first right?
such

ω = Θ/t

so later I do the other equation.

is this correct?

6. Jun 11, 2014

### gcombina

so to solve for ω, i have

ω = Θ/t
= 20 (2∏) / (what should I put here in time if time wasn't given)

7. Jun 11, 2014

### dauto

Use the equation for angular displacement in an angular accelerated motion. You don't have to waste time calculating angular velocities

8. Jun 11, 2014

### Nathanael

That would be average (not final) angular velocity.

Imagine the graph of the angular velocity over time. What is special about that graph? How can you use that to create an equation for the angular displacement?