# Homework Help: Spinin wheel

1. Apr 28, 2005

### la673

At time t=0 a grinding wheel has an angular velocity of 29.0 rad/s. It has a constant angular acceleration of 34.0 rad/s^2 until a circuit breaker trips at time t = 2.30 s. From then on, the wheel turns through an angle of 434 rad as it coasts to a stop at constant angular deceleration.

now i've gotten teh total angle the wheel turned between t=0 and the time it stopped, but i cant work out
a) what tiem the wheel stops; or
b) the angular deceleration after 2.3 s

i tried using omega = omega_0 + alpha_deceleration* t and 2*alpha_deceleration* (theta-theta_0) = omega^2-omega_0^2, using final omega = 0.

but this ended up with an end time of 315.3 seconds.... which was wrong...

help if u can plz

2. Apr 28, 2005

### OlderDan

What did you get for the maximum omega of the wheel?

3. Apr 28, 2005

### la673

107.2, usin omega=omega_0+alpha_o*t

4. Apr 28, 2005

### OlderDan

That looks about right. Now you know how many radians it turns to come to rest, and you know its initial angular velocity and its final angular velocity. When angular acceleration is constant it is very easy to find average angular velocity, and by definition average angular velocity is ....?

5. Apr 28, 2005

### la673

change in angle/ change in time, but i cant figure out after what time it stops.

6. Apr 28, 2005

### la673

wiat, os 14.78= 156.63/change in time?

7. Apr 28, 2005

### la673

nd so the chnage in time is 10.6 s after the intital 2.3 s?

8. Apr 28, 2005

### la673

nd so by using total time is 12.9s, u use the forumla omega_max=omega_intital+alphja*t to find the angular deccelration?
so 107.2=29+alpha_deceleration*12.9, so alpha is around 6.06? plz tell me if this look right, or am i doin it all wrong?

-edit: lol srry didnt even notice teh edit buttons

nd do u change the 6.06 to a negative to show its deceleration?

Last edited: Apr 28, 2005
9. Apr 28, 2005

### OlderDan

Can't quite follow your work on this, but it is not right. It appears you may be trying to do things with the total time for speeding up and slowing down. At this point you should be looking at just the slowing down process. Go back to the maximum angular velocity of 107.2 rad. That part is good. The deceleration from that velocity is assumed to be constant. When you have constant acceleration, the average velocity over the time interval is the average of the velocities at the beginning (107.2) and the end (stopped). When you know the average vlocity, you apply to what you said about average velocity

and solve for the time. Once you have the time, you can find the angular deceleration.

There is aso an equation that gets to the result for the acceleration without calculating the time, but you are asked for the time, so this is the approach you want.