Rotational motion of a computer hard disk

[sktgurl930]

Homework Statement

A computer hard disk starts from rest, then speeds up with an angular acceleration of 198 rad/s2 until it reaches its final angular speed of 7683 rpm. How many revolutions has the disk made 10.7 s after it starts up?

Homework Equations

t=wf/a
delta0=wi*t+(1/2)(a)t^2

The Attempt at a Solution

when using the first equation i got t=804.561878/198=4.06
then i plugged it into the second equation which would be delta0=0*4.06+(.5)(198)(6.63655)^2
=4.06+99*44.04
=4364.4039

but it says that its wrong i don't understand what I am doing wrong

 

[LowlyPion]

Homework Statement

A computer hard disk starts from rest, then speeds up with an angular acceleration of 198 rad/s2 until it reaches its final angular speed of 7683 rpm. How many revolutions has the disk made 10.7 s after it starts up?

Homework Equations

t=wf/a
delta0=wi*t+(1/2)(a)t^2

The Attempt at a Solution

when using the first equation i got t=804.561878/198=4.06
then i plugged it into the second equation which would be delta0=0*4.06+(.5)(198)(6.63655)^2
=4.06+99*44.04
=4364.4039

but it says that its wrong i don't understand what I am doing wrong

Too much precision?

 

[LowlyPion]

Homework Statement

A computer hard disk starts from rest, then speeds up with an angular acceleration of 198 rad/s2 until it reaches its final angular speed of 7683 rpm. How many revolutions has the disk made 10.7 s after it starts up?

Homework Equations

t=wf/a
delta0=wi*t+(1/2)(a)t^2

The Attempt at a Solution

when using the first equation i got t=804.561878/198=4.06
then i plugged it into the second equation which would be delta0=0*4.06+(.5)(198)(6.63655)^2
=4.06+99*44.04
=4364.4039

but it says that its wrong i don't understand what I am doing wrong

Up to speed means 7683 RPM = 128.05 rps
In radians that's 804.53
Divide that by 198 and that equals the 4.06 s
How many radians is that? 1/2*198*(4.06)² = 1631.88
Now how many more at max speed.
10.7 - 4.06 = 6.64 times 804.53 = 5342.01
Add the two = 6973.96

 

[sktgurl930]

im sry for asking this but y did u times 6.64 by 804.53?? and then add the two answer

 

[LowlyPion]

im sry for asking this but y did u times 6.64 by 804.53?? and then add the two answer

You have an interval of 10.7 seconds. The first 4.06 seconds was accelerating to max speed. So you need to calculate the number of radians it took to do that.

But then it kept on spinning at the max speed didn't it? And that 6.64 seconds is how long it spun at max speed.

So ... you calculate the number of additional radians until the time budget is expired. The two together is the total number of radians.

Remember they were asking for revolutions so you still have that conversion.