# Homework Help: Constant angular acceleration problem

1. Nov 4, 2007

### saturn67

A)If it took 0.480 s for the drive to make its second complete revolution, how long did it take to make the first complete revolution?

B)What is its angular acceleration, in rad/s^2?

2. Nov 4, 2007

### G01

OK, in order for to give you some help, I am going to need to things. I need to see some of your work, following the rules of the forum, and I'll need the whole question. It doesn't seem like you supplied enough information here.

If you don't have any calculations to show yet, at least point out the formulas and concepts that you think apply, etc. You must know something about the problem.

3. Nov 4, 2007

### saturn67

does this Equation work?
deta-deta.not = (1/2)(w2+w1)t in order to find t

since we know total angle

4. Nov 4, 2007

### G01

What is deta-deta.not, and what is the full question??

5. Nov 4, 2007

### saturn67

(anglefinal - anglebeginning) = (1/2)(omega2+omega1)*t

6. Nov 4, 2007

### G01

That is a correct formula, but again, I can't tell you if it will be useful or not if you don't give me the whole question. It seems obvious that there is more to the problem. In part A, you make reference to "the" drive, thus assuming it has been mentioned before, and as far as I can tell, there is not enough information given in what you posted to solve the problem.

7. Nov 4, 2007

### saturn67

The computer disk drive is turned on starting from rest and has constant angular acceleration.

A)If it took 0.480 s for the drive to make its second complete revolution, how long did it take to make the first complete revolution?

that all it say in the book :(

8. Nov 4, 2007

### G01

AHH ok! The drive starts spinning at rest. The formula you supplied last time should work, but I think you may want to consider using another (see hint).

HINT: Finding the acceleration first, from the information from the first rotation may be beneficial. Can you give me an equation relating the angular acceleration to the speed, angular displacement and time of the first rotation? You should then be able to use that acceleration to help you find the answer to part A.

9. Nov 4, 2007

### saturn67

i'm not sure but

omega2=omega1+a*t

or

omega2^2=omega1^2 + 2*a*(angle2-angle1)

10. Nov 5, 2007

### G01

Yes, you can use both of those to find the angular acceleration.

Can you find it now using one of these formulas and the information about the first rotation?

11. Nov 5, 2007

### tmle04

what is t in that equation??? time after 2nd revolution or 1st revolution. also how can i solve it when it did not give the angular acceleration???

12. Nov 5, 2007

### saturn67

(anglefinal - anglebeginning) = omega1*t+(1/2)a*t^2

t=0.480 s

4pi = 1/2*a*(0.480)^2

8pi/(0.480)^2 = a

still give me wrong answer for angular acceleration :(

13. Nov 5, 2007

### saturn67

can someone help me please . seem like GO1 offline -_-'

14. Nov 5, 2007

### tmle04

whered da 4pi come from???

15. Nov 5, 2007

### saturn67

can someone tell me why it give me wrong answer for angular acceleration?

16. Nov 5, 2007

### tmle04

it should be......

-4pi*t = 1/2*a*(0.480)^2....

now tell me wats ur time after one revolution????and how did u get it???

17. Nov 5, 2007

### saturn67

i need to find a first than find time after that

18. Nov 5, 2007

### tmle04

ignore da above comment...........it should be......

-(2/.480)2pi*t = 1/2*a*(0.480)^2....

now tell me wats ur time after one revolution????and how did u get it???

19. Nov 5, 2007

### saturn67

(anglefinal - anglebeginning) = omega1*t+(1/2)a*t^2

omega1 = 0 because it start from rest

4pi = 2 rev

20. Nov 5, 2007

### saturn67

what equation u using ?