Finding Rotational Energy and Acceleration of Decending Mass

[nvallettejr]

[b]1. A disk with radius of 0.3m and a mass of 4kg is free to rotate about an axle through its center. A cord wrapped around the circumference supports a weight with mass 5kg.

a) What is the acceleration of descending mass if it is released?

b) 5 seconds after release what is the rotational energy of the disk?



[b]2. I disk = (1/2)MR2


[b]3. I know that Idisk = (1/2)(4)(.3)2 is 0.18. After that I am lost.

[kuruman]

Draw two free body diagrams, one for the disk and one for the hanging mass. Write Newton's 2nd Law pertinent to each FBD.

[nvallettejr]

Okay that sounds right I have that done but how do we get the rotational energy after 5 seconds??

[kuruman]

If you know the angular acceleration, you can get the rotational speed at t = 5 s from the kinematic equations and from that the rotational energy.