Is holding something in a gravitational field doing work ?

[atyy]

And that's even if we let atyy's objection slide: the work done on the ball doesn't come from dad's arm, it comes from the ground pushing up on dad's feet.

That's not my objection. The direct contact comes on the ball comes from the hand that is holding it, so we can assign the force to that, rather than working in 3rd law pairs all the way to the ground.

I agree with DaleSpam that it is the net force that does work, and that if we wish to assign work done to various force components, then it is reasonable to assign the work done to the "inertial force", and not to the man. The "inertial force" is distinguishable because it is not part of a 3rd law pair. This becomes especially clear if we use an accelerated frame that is not a free-falling frame, eg. the upward accelerating frame Phrak suggests. (Minor point: DaleSpam and I use different terms for the free-falling frame - I call it an accelerated frame, working within Newtonian theory, DaleSpam calls it an inertial frame, working from GR - not quite true since Newton knew the Principle of Equivalence - anyway, that's just nomenclature.)

I think the weasling is great! Just like school kids should learn that velocity is meaningless without the specification of a reference frame, they should learn the same for work and energy. Furthermore, I think the weasel is adequately punished since it can't even win the bet if we concede to its terms (The claim was not "Work is being done", but "I am doing work").

Of course, kids should also be taught to give the "right" answers on exams - that's life - the boss is always right (unless you're being asked to do something morally wrong).

 

[sp1408]

Think about it this way...the person is holding something in air by applying some force on it...If there was no gravity,and he would still apply the same force,the body would accelerate and you would be doing work on it...of course this is a bit difficult to visualize in the sense that you wouldn't need to hold anything if there was no gravity...

 

[Topher925]

Wow, way to take a simple problem and make it 100x times more complicated than it needs to be. This is why I am an engineering and not a physicist. Anyways...

..If there was no gravity,and he would still apply the same force,the body would accelerate and you would be doing work on it...

So your saying that all mass is constantly doing work regardless of what state or properties it might have? I'm not sure but I think there's something wrong with that logic.

 

[ValenceE]

Post # 59, Dalespam writes "The whole point is that energy and work are not intrinsic quantities, "

are you saying that any and every reference frame doesn't have a minimal energy quantity ?

VE

 

[Dale]

What is "a minimal energy quantity"?

 

[ValenceE]

DaleSpam,

I guess I’m referring to zero-point energy. I do believe that space-time is permeated with energy and that there is a minimal energy quantity or density that must be above zero.

In this view, isn’t energy intrinsic to space-time, whatever the coordinates, whichever reference frame is selected?


Regards,

VE

 

[Dale]

I don't know much about zero-point energy. But my understanding is that it is a prediction of the standard model and the standard model is compatible with SR, so I would be surprised if it weren't properly frame-variant.

Do you have some reason to think that it isn't?

 

[atyy]

In this view, isn’t energy intrinsic to space-time, whatever the coordinates, whichever reference frame is selected?

I don't know much about zero-point energy. But my understanding is that it is a prediction of the standard model and the standard model is compatible with SR, so I would be surprised if it weren't properly frame-variant.

I suppose VE's point is that a cosmological constant or vacuum energy would not only be properly frame-variant, but would also be frame-invariant, unlike Newtonian kinetic energy and work. I'm a bit confused this when we go out of Newtonian physics, but it seems we don't even have to go to GR to make sense of VE's point. In SR, E²=p²c²+m_o²c⁴ is the squared length of a four-vector, so it is frame-invariant, although p is frame-variant. In Newtonian physics, work and kinetic energy are useful because of the work energy theorem, and because KE+PE is conserved for curl free force fields. In SR, E is defined differently from Newtonian physics, because physicists can't bear the thought that energy is not conserved, and by demanding conservation, one also obtains invariance.