# Something weird about Rotational Kinematic Equations

• Stefenng
In summary, the average acceleration of the gramophone record can be found using the equation a = (w - W)/t, where t is the time it takes for the record to rotate from an initial angular velocity, W, to a final angular velocity, w, while undergoing an angular displacement of 8. The resulting answer is 49.0 rad s^-2. The attempt at a solution using the equation w2 = W2 + 2a8 is incorrect because the angular velocity, w, is not constant. The correct equation is

## 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

## Homework Equations

The equation given by the answer is

t= 8/w
a = ( w - W ) / t

## The Attempt at a Solution

This is the equation I use

w2 = W2 + 2a8

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?

Stefenng said:

## Homework Equations

The equation given by the answer is

t= 8/w
a = ( w - W ) / t

## The Attempt at a Solution

This is the equation I use

w2 = W2 + 2a8

You are right:

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

It should be t= 28/w

ok...thx... ^^

Stefenng said:

## Homework Equations

The equation given by the answer is

t= 8/w
a = ( w - W ) / t

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

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