Something weird about Rotational Kinematic Equations

[Stefenng]

Homework Statement

I need to find the average acceleration of a gramophone record.

Initial angular velocity, W = 0
Final angular velocity, w = 3.50rad s^-1
angular displacement, 8 = 0.25rad

Homework Equations

The equation given by the answer is

t= 8/w
a = ( w - W ) / t
the answer is 49.0 rad s^-2

The Attempt at a Solution

This is the equation I use

w² = W² + 2a8

and I get 24.5rad s^-2

But if I multiply it with 2, the angular acceleration will become 49rad s^-2

That mean my equation will become
w² = W² + a8

Can anyone explain this to me?
Or I use a wrong equation?

 

[davieddy]

Homework Equations

The equation given by the answer is

t= 8/w
a = ( w - W ) / t
the answer is 49.0 rad s^-2

The Attempt at a Solution

This is the equation I use

w² = W² + 2a8

and I get 24.5rad s^-2

You are right:

t= 8/w is wrong because the angular velocity is not constant

It should be t= 28/w

 

[Stefenng]

ok...thx... ^^

 

[davieddy]

ok...thx... ^^

THX for the THX
Try this:
http://www.youtube.com/watch?v=ElNpwX_8YpQ&feature=related

 

[tiny-tim]

Homework Equations

The equation given by the answer is

t= 8/w
a = ( w - W ) / t
the answer is 49.0 rad s^-2

Hi Stefenng!

(have a theta: θ and an omega: ω )

You must learn the standard https://www.physicsforums.com/library.php?do=view_item&itemid=204" equations …

it isn't θ = ωt (or t = θ/ω), it's θ = ωt + 1/2 αt²