Constant angular acceleration problem

[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?

please help

 

[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.

 

[saturn67]

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

since we know total angle

 

[G01]

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

 

[saturn67]

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

 

[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.

 

[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 :(

 

[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.

 

[saturn67]

i'm not sure but

omega2=omega1+a*t

or

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

 

[G01]

i'm not sure but

omega2=omega1+a*t

or

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

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?

 

[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?

 

[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

a= 109.083 rad/s^2

still give me wrong answer for angular acceleration :(

 

[saturn67]

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

 

[tmle04]

whered da 4pi come from?

 

[saturn67]

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

 

[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?

 

[saturn67]

i need to find a first than find time after that

 

[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?

 

[saturn67]

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

omega1 = 0 because it start from rest

4pi = 2 rev

 

[saturn67]

what equation u using ?

 

[tmle04]

iam using ur equation...it should be 2 rev per .480 second...then u time 2 pi...do u go to asu?

 

[saturn67]

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

if u try to solve for t

u need to find a first

which i try to solve for a

i don;t think ur EQ will work
because u have t and a in one EQ

 

[tmle04]

i don't kno I am confuse...anyway i kno omega2 = (2/0.480)*2pi

 

[saturn67]

i'm confuse too :(

any expert around here please help us

 

[tmle04]

my roomate is helping me right now...he did dis already...ill let u kno when i figure it out...

 

[saturn67]

Thank you i hope he good XD, tell me right anyway after u done

 

[tmle04]

ur equation is correct... i don't kno why

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

mastering physics accepteed his answer but mine...

 

[saturn67]

werid

it don;t accept mine

 

[saturn67]

what his time?

0.339 sec?

 

[rl.bhat]

Angular displacement = wo + 1/2*a*t^2. Since you are talking about first and second rotation , it must be starting from rest. Therefore wo = 0.
Let it take t second to complete first rotation and t + 0.48 s for two rotation. So 2pi = 1/2*a*(t)^2 and 4pi = 1/2*a*(t + 0.48)^2 . Divide these two equations. a gets canceled out. Solve for t. Put this value in the first equation. You will get the value of a.

 

[saturn67]

how to Divide these two equations to get a cancel ?

 

[rl.bhat]

2pi/4pi = 1/2*a*t^2/1/2a*(t+0.48)^2
1/2 = t^2/(t+0.48)^2
solve for t

 

[saturn67]

that give t=0.48
which it like not solving anything :(

 

[rl.bhat]

Cross multiplying the above expression we get (t + 0.48 )^2 = 2t^2
Taking sqrt. on both side we get t + 0.48 = 1.41t or 0.41t = 0.48 or t = 0.48/0.41

 

[saturn67]

woot u were right
LOL stupid me
thank you