# Rotational Motion of a disk

A disk with a radial line painted on it is mounted on an axle perpendicular to it and running through its center. It is initially at rest, with the line at q0 = -90°. The disk then undergoes constant angular acceleration. After accelerating for 3.1 s, the reference line has been moved part way around the circle (in a counterclockwise direction) to qf = 153°.
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since theta = theta(0) + omega(0)t + 0.5*alpha*t^2, for theta = angle, omega = angular velocity, alpha = angular acceleration, and t in seconds,
153 = -90 + 0*t + 0.5*alpha*t^2
243 = 0.5*alpha*t^2
alpha = 486/t^2
and so alpha should be around 50.5723205... right?!

but it's not!
the degrees seem right, because 243 degrees is the degree of difference. since the disk was initially at rest, omega(0) = 0.
what am i doing wrong?

// got rid of latex cause it wasn't working for some reason

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## Answers and Replies

Maybe, the scale goes clockwise so your disk rotates -90 to -180 then 180 to 153, causing a total change of 117 degrees instead of 243. Let me know if that gives you the answer you're looking for...

Regards,
Sam

nahya said:
A disk with a radial line painted on it is mounted on an axle perpendicular to it and running through its center. It is initially at rest, with the line at q0 = -90°. The disk then undergoes constant angular acceleration. After accelerating for 3.1 s, the reference line has been moved part way around the circle (in a counterclockwise direction) to qf = 153°.
---
since theta = theta(0) + omega(0)t + 0.5*alpha*t^2, for theta = angle, omega = angular velocity, alpha = angular acceleration, and t in seconds,
153 = -90 + 0*t + 0.5*alpha*t^2
243 = 0.5*alpha*t^2
alpha = 486/t^2
and so alpha should be around 50.5723205... right?!

but it's not!
the degrees seem right, because 243 degrees is the degree of difference. since the disk was initially at rest, omega(0) = 0.
what am i doing wrong?

// got rid of latex cause it wasn't working for some reason

Normally in these problems the angles are expressed in radians, not degrees. The method looks good, anyway.

-Dan

I would be surprized if they quoted the angles in degrees and then expected an answer in radians, but its a posibility.

Sam

BerryBoy said:
I would be surprized if they quoted the angles in degrees and then expected an answer in radians, but its a posibility.

Sam

Believe me, it's happened before! :yuck:

-Dan

omg... they DID want it in radians..................
hahaha.. i'm sorry. i wasn't careful in reading the problem.
(in the answer box it says rad/s^2. bleh)