Calculating Rotational Inertia: Hoops vs. Solid Cylinders

[goonking]

Homework Statement

Homework Equations

The Attempt at a Solution

I assume the energy stored is = 1/2 (I) (ω^2)

I (moment of inertia) is MR^2 since it's a hoop? or is it a solid cylinder?

do we need to convert the rpm (revolutions per minute) to radians per sec?

 

[B0b-A]

Homework Statement

I (moment of inertia) is MR^2 since it's a hoop? or is it a solid cylinder?

The question says "disc" ... http://hyperphysics.phy-astr.gsu.edu/hbase/tdisc.html

 

[MaxwellsCat]

You're on the right track, just realize that a disc is a cylinder with vanishing height.

Otherwise yeah, try it out and report back on what you end up with. It'll be important to do a sanity check on the answer that you get - that'll tell you whether you got the right answer or not most of the time.

 

[AlephNumbers]

You're on the right track, just realize that a disc is a cylinder with vanishing height.

No, that's a thin disk. A disk is a solid cylinder.

do we need to convert the rpm (revolutions per minute) to radians per sec?

As you defined it, ω is angular velocity, which is typically measured in radians per second.

 

[MaxwellsCat]

Sorry, I should have been explicit - I meant a solid cylinder.

 

[goonking]

after doing all the math, answer came out to be 1.4 x 10^6 Js.

 

[MaxwellsCat]

So right away there's something wrong - the units should be J not J$\cdot$s, unless you meant Joules :P

Does that answer make sense? In the context of the problem, does that seem reasonable?

 

[goonking]

So right away there's something wrong - the units should be J not J$\cdot$s, unless you meant Joules :P

Does that answer make sense? In the context of the problem, does that seem reasonable?

yes, i meant joules.

 

[haruspex]

after doing all the math, answer came out to be 1.4 x 10^6 Js.

Seems too low. Check your conversion from revolutions to radians.