Energy and non-conservative forces

[Josh0768]

Homework Statement

When accounting for the work done by non-conservative forces in conservation of energy problems, how can you tell which side of the equation that work needs to go in?

Relevant Equations

K - W = U
OR
K = U - W
??

I feel like it would go on the side of the energy the object has where it starts - an object dropped off a cliff would be modeled U - W = K but an object thrown upwards from ground level would be
K - W = U. I am not sure though.

 

[PeroK]

Problem Statement: When accounting for the work done by non-conservative forces in conservation of energy problems, how can you tell which side of the equation that work needs to go in?
Relevant Equations: K - W = U
OR
K = U - W
??

I feel like it would go on the side of the energy the object has where it starts - an object dropped off a cliff would be modeled U - W = K but an object thrown upwards from ground level would be
K - W = U. I am not sure though.

I never quite understand these questions. It's a bit like asking: "if I have a finanicial transaction, when do I subtract it from my bank account and when do I add it?"

Normally these force are dissipative, so they reduce the total energy of the system. But, you could have external forces (like wind power) that add energy to a system.

Equations are generated by the physical aspects of the problem. If you understand the problem physically, then there should be no difficulty in generating the correct equation.