Some help please?

Jeslek

Jeslek

Banned
I'm getting muddled with the units... Here is the question.

Define the moment of inertia of a planet I. The spin energy of a planet that rotates with angular velocity w is Iw²/2. For the Earth, this changes because the spin changes at a rate of approximately 2 msec/century. Calculate the loss of energy of the Earth due to its spindown.

You can take I = Mr²/3 where M = 6.0 x 10^24 kg, and r = 6.37 x 10^6 m.

Express your answer in watts.



Now lets see. Watts is defined as kg·m²/sec³. Joule is defined as kg·m²/sec² since watts is also J/sec.

I = Mr²/3 = <some number> kg·m².

Now I'm stuck. How do I express angular velocity? And the change in w is 2 msec/century. Now we have to convert it to standard units, so then you end up with <some number> sec/sec. This unit is essentially dimensionless.

H E L P.
 
moment of interia are determined using integrals though

you may have to go back a step and get an equation with change in angular velocity in ti
 
Maybe your physics course is different. Ours is a calc based course...so pretty much everything involves calc...:(
 
See, the thing is, you can manipulate some of the equations.

If we take E = the spin energy of a planet to be joules (kg·m²/sec²), then

E = Iw²


and w² = 2E/I = kg·m²/sec²/kg·m² = 1/sec²

So it seems I need a quantity with one dimension in time. :eh:
 
Would ya rather apply yer effort to tryin' to answer the question or knowin' the one who has the answer?..................(sounds like Nix is best able to assist 'ere............?(
 
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