Calculating the Rotational Inertia

Doc Al
Mentor
I double-checked the calculation: it's good. What exactly did you enter?

Stupid question: Since the problem gives a rotational speed, are you sure they are asking for rotational inertia and not angular momentum? (Is it a multiple part question?)

I entered 0.07695 and I don't have to include units because webassign doesn't take them. I also enter 0.077.

The problem gives me the period, like you said, which is messing me up. The first question is:

(a) Assuming R = 0.90 m and m = 3.0 kg, calculate the structure's rotational inertia about the axis of rotation.

The second question is:

(b) What is its angular momentum about that axis?

I know that angular momentum is defined by the moment of inertia multiplied by the angular velocity.

I am not sure how I would go about solving part (b) to get part (a), although that would be great.

Doc Al
Mentor
0.9m or 0.09m???

(a) Assuming R = 0.90 m and m = 3.0 kg, calculate the structure's rotational inertia about the axis of rotation.
Is the radius 0.9m or 0.09m? You've been using 0.09m in your calculations. That could explain a lot! :uhh: (I see that your original post stated 0.9m--sorry for not spotting that earlier.)

Wow... speechless. That was it, off by a factor of ten. Final answer = 7.695 kg*m^2

I tell ya, sometimes you get so caught up in trying to comprehend a problem that you forget what the problem actually stated. Thank you so much, Doc Al for your patience and help.