Comparing Energy Transfer and Speed Changes in Isolated Systems

[Amik]

Homework Statement

I just want to know how to choose a isolates system

Relevant Equations

Isolated system

I have difficulty at choosing isolated system.We can not choose rope and block to be the isolated system?This is for number 20.

 

[berkeman]

No clue what you mean by isolated system. Draw the two FBDs for the block and the cylinder.

Also, it looks like you deleted the section on the Relevant Equations. What is the Relevant Equation for the MOI of a mass? And the related Relevant Equation for the angular acceleration of such a mass with the calculated MOI?

 

[Amik]

I am choosing to use the conservation of energy to solve the problem.

 

[gneill]

I am choosing to use the conservation of energy to solve the problem.

How so? What's your plan of attack? I believe it is doable, but I'd like to see your approach.

 

[berkeman]

I am choosing to use the conservation of energy to solve the problem.

COE to solve a MOI problem? I wouldn't think that's possible, but @gneill hints that it might be possible, and I trust him...

Okay, I see it now, but that wouldn't be my first approach to this fairly simple problem. Just sayin'

 

[Amik]

I know using moment of inertia is way easier and this problem is in the section of the conversation of energy

 

[gneill]

I know using moment of inertia is way easier and this problem is in the section of the conversation of energy

You'll need the MOI anyways in order to determine how the energy is split between rotational and linear kinetic energies.

 

[Amik]

Yeah, I know.

 

[gneill]

Okay. While the fixed axle of the cylinder is clearly tying the system into a "greater" outside system, you should be able to proceed as though the cylinder and dangling mass are isolated as far as energy is concerned. What that axle is tied to can be taken as so massive that no appreciable momentum or energy will be transferred there. So what's your plan?

 

[kuruman]

What that axle is tied to can be taken as so massive that no appreciable momentum or energy will be transferred there.

I think what you want to say is that the transfer of energy that takes place changes appreciably the rotational speed of the cylinder and the linear speed of the hanging mass but not the corresponding speeds of whatever the axle is tied to. The energy gain by one has the same magnitude as the energy loss by the other. To determine whether the energy transfer is negligible, it needs to be compared against an energy other than itself. It's the speeds that need be compared, not the energy transfer.

 

[gneill]

I think what you want to say is that the transfer of energy that takes place changes appreciably the rotational speed of the cylinder and the linear speed of the hanging mass but not the corresponding speeds of whatever the axle is tied to. The energy gain by one has the same magnitude as the energy loss by the other. To determine whether the energy transfer is not appreciable, it needs to be compared against another energy other than itself. It's the speeds that need be compared, not the energy transfer.

Yes, that's a much better description of what I was intending. I sometimes find I'm too curt in my responses. Thanks for the assist.