# Homework Help: Some questions concerning centripetal force.

1. Dec 11, 2006

### wtf_albino

1. A mass attached to a 50-cm long string starts from rest and is rotated 40 times in 1.00 min before reaching a final angular speed. Determine the angular acceleration of the mass, assuming that it is constant.

W = Delta Theta/Delta Time
Average Acceleration = Delta W/Delta time
W = Wo + at
Theta = Wo(t) + 1/2at^2
W^2 = Wo^2 + 2a(Theta)

3. I started by finding W. It said that the wheel made 40 revolutions in 1 minute so i put:

40 Rev/60 s : 2(pi)/rev

and from this i got: 4.2 Rad/s

Using this i used the formula W = Wo + at:

4.2 = 60a
.07 rev/s^2 = A

the answer in the book says .14 rad/s^2 can anyone point out my mistake?

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 11, 2006

### carbz

I would suggest to check your units. Since you're finding acceleration, the units must match to a unit of acceleration. Please check on that.

3. Dec 11, 2006

### Staff: Mentor

Unless the angular speed is constant, this gives you average speed.

That's the average angular speed, not the final speed.

This equation requires the initial and final angular speeds, not average speed.

Hint: Since the angular acceleration is uniform, there is a simple relationship between the average speed (which you calculated) and the initial and final speeds. What is it?

4. Dec 11, 2006

### wtf_albino

hm.. I read through the section covering this part but i couldn't get anything.. could i get another hint?

5. Dec 11, 2006

### carbz

Try to remember the formulas that you learned. There are two formulas that you could use, although there is one that is prefered.

6. Dec 11, 2006

### Staff: Mentor

Last edited by a moderator: Apr 22, 2017
7. Dec 11, 2006

### wtf_albino

thanks alot doc and carbz, ill definitely check it out.

8. Dec 11, 2006

### wtf_albino

sorry, i'm still not seeing it.

9. Dec 12, 2006

### Staff: Mentor

There are two ways to do this problem:
(1) You can continue the way you started, once you figure out the final speed. That list of kinematic equations has one that relates initial and final speed to average speed.
(2) You can make use of a different kinematic formula, one that directly relates distance, time, and acceleration. That too is on the list I provided.