Rotational Dynamics Homework Help

[lc99]

Homework Statement

Homework Equations

RKE = .5iw^2
TKE = .5mv^2
U = mgh

The Attempt at a Solution

is my setup correct?

mgh = RKE +TKE
mgh = .5Iw^2 + .5mv^2

do i just total up the RKE and TKE for both the objects?[/B]

 

[Khashishi]

This question is vague. It doesn't say whether to include the potential energy in the result. Nevertheless, you have to press onward and guess what they want. They probably want the kinetic energy of mass 2 at the moment it is 10 cm below the starting point.

To do this, you should write the initial and final energies and set them equal to each other. The initial energy can simply be set to 0.
The final energy will consist of: potential energy of the two masses, kinetic energy of the two masses, rotational kinetic energy of the shaft. Write them out fully.

 

[haruspex]

is my setup correct?

Your attempt is too vague to be sure.
E.g. your energy equations look like they are for a single isolated mass, but here there are three masses interacting.
Please post further work.

 

[haruspex]

They probably want the kinetic energy of mass 2 at the moment it is 10 cm below the starting point.

I agree.

 

[lc99]

This question is vague. It doesn't say whether to include the potential energy in the result. Nevertheless, you have to press onward and guess what they want. They probably want the kinetic energy of mass 2 at the moment it is 10 cm below the starting point.

To do this, you should write the initial and final energies and set them equal to each other. The initial energy can simply be set to 0.
The final energy will consist of: potential energy of the two masses, kinetic energy of the two masses, rotational kinetic energy of the shaft. Write them out fully.

How come the intial energy is 0? there is gravitational potential energy though

 

[Khashishi]

The potential energy has an arbitrary choice of zero. You can set it to anything you want. 0 was just a suggestion.