# Rotational Kinematics Question

#### sophixm

1. The problem statement, all variables and given/known data
A computer disk drive is turned on starting from rest and has constant angular acceleration.
If it took 0.680s for the drive to make its second complete revolution, how long did it take to make the first complete revolution? (t=?)
2. Relevant equations
Θ=(ct^2)/2 (i think)

3. The attempt at a solution
So, since the angular acceleration is constant, I'm assuming α=c (c being a constant).
So then the intergral with respect to time, would be ω=ct, and then Θ=(ct^2)/2
I tried solving for c using 4π(2 revolutions)=(c(.68^2))/2, and got 54.4 for c, and used that to solve for time of one revolution, but this is wrong. I guess what the problem is saying is that it took 0.680 seconds to go from 2π to 4π, so im stuck

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#### PeroK

Homework Helper
Gold Member
2018 Award
1. The problem statement, all variables and given/known data
A computer disk drive is turned on starting from rest and has constant angular acceleration.
If it took 0.680s for the drive to make its second complete revolution, how long did it take to make the first complete revolution? (t=?)
2. Relevant equations
Θ=(ct^2)/2 (i think)

3. The attempt at a solution
So, since the angular acceleration is constant, I'm assuming α=c (c being a constant).
So then the intergral with respect to time, would be ω=ct, and then Θ=(ct^2)/2
I tried solving for c using 4π(2 revolutions)=(c(.68^2))/2, and got 54.4 for c, and used that to solve for time of one revolution, but this is wrong. I guess what the problem is saying is that it took 0.680 seconds to go from 2π to 4π, so im stuck
Take a step back. If something accelerates from rest and takes $t_1$ seconds to travel an angle $\theta$, then how long does it take to travel a further $\theta$?

#### haruspex

Homework Helper
Gold Member
2018 Award
the problem is saying is that it took 0.680 seconds to go from 2π to 4π
Quite so. Are you familiar with the SUVAT equations for constant linear acceleration? The equations for constant rotational acceleration are strongly analogous.

#### sophixm

Quite so. Are you familiar with the SUVAT equations for constant linear acceleration? The equations for constant rotational acceleration are strongly analogous.
I believe so, ones like Θ=Θinitial+ωinitial(t)+(1/2)αt^2 right? Ive taken some time to look at those but I'm still stuck. I'm familiar with the method of subbing in when you have two unknown variables, but i feel like I'm missing more that that. All I know is that at a time it has gone 2π radians, and 0.680 seconds from that time it has gone another 2π radians. I feel like I'm just missing something

#### haruspex

Homework Helper
Gold Member
2018 Award
I believe so, ones like Θ=Θinitial+ωinitial(t)+(1/2)αt^2 right? Ive taken some time to look at those but I'm still stuck. I'm familiar with the method of subbing in when you have two unknown variables, but i feel like I'm missing more that that. All I know is that at a time it has gone 2π radians, and 0.680 seconds from that time it has gone another 2π radians. I feel like I'm just missing something
Let ti, i = 0, 1, 2, be the time at which the ith rotation is completed (t0=0, of course). Likewise three 'distances' $\theta_0, \theta_1, \theta_2$, one acceleration. What equations can you write down involving these?

"Rotational Kinematics Question"

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