Why can energy be positive or negative if it's the capacity to do work?

[MrNerd]

If there is no motion, there is no energy requirement, as per w=fd. If d=0, w=0, regardless of what f is.

Alright then. Just to be clear, though, when you say no energy requirement, do you mean no net difference between energy levels? I'm thinking of the second law of thermodynamics here. Would a high relative energy cause a force on a low relative energy only if there is motion/change in position(using W = Fd, not things like nuclear energy)?

 

[Dale]

The second law of thermo doesn't say anything about force.

 

[MrNerd]

Ok, then. I'll think I'll just wait until the upper classes in college for those kinds of things.

 

[galtsgulchb]

Hmmmm... I was sure it was. ...Could you explain which forces don't need energy to operate, please?

It would be easier to answer a more specific question since I don't quite follow what you mean by a force that needs energy or operate. Let me try to clarify the relationship between force, energy, work, and distance.

Imagine a cylinder filled with gas at pressure 'p' pushing on a piston. This means that there is potential energy stored in the gas so it is capable of doing work. The force applied to the piston is, F=p*A. Now let's consider two cases: [1] the piston does not move and [2] the piston moves.

[1] the piston does not move. If I am holding the piston from the other side so that it can't move then no work is done since W=Fd. This means that none of the potential energy from the gas is transferred to the piston. However, it is worth noting that there is a force applied due to the potential energy in the gas. So in this sense you are correct, the force was due to the energy stored in the gas. However, what physicist are usually interested in is whether energy is transferred (work done) and here it was not so W=0.

[2] the piston moves. This is the more interesting case. If I let go of the piston it now moves some distance and work is done. This means that the energy is transferred from the gas to the piston and the work done is W=Fd.

 

[nouveau_riche]

You should be self-aware enough to accept that when multiple people have trouble with your use of a pretty basic concept, it's probably not everyone else who has the problem, but you.

probability not certainity

 

[nouveau_riche]

Obviously we cannot "count" your presence if we are only allowed to infer your presence due to motion of the wall and you do not move the wall. So what? That is entirely due to the restriction that we are only allowed to infer your presence from the motion of the wall, and has nothing directly to do with energy.

Why do you consider that at all relevant or important to a discussion on energy?

well think more and you will get a change in perception to your "so what"

 

[Dale]

So you don't have any actual logical reason why your scenario is either relevant or important to a discussion on energy.

 

[BruceW]

nouveau - does galtsgulchb's post at the end of the last page answer your question?