riconex's Object Lifting Problem

-What's weird about weight as a force-

This question was taken from Khan Academy's comment section. I was watching a video to figure about The Strange PE of Elliptical Orbits, and I found this. Being strangely counterintuitive actually brings one important property of weight to light.

Hmmm... if no work is done, certainly the object cannot gain energy no? Or consider a car moving at constant speed. No net work done, no energy change.

(excuse my cuboidal car)

Isn't the car situation basically the same with the lifted object?

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My answer for now...

Weight is a special force. It doesn't change the mechanical energy (KE + GPE) of the system. Unlike resistance (from the car example), weight does not remove energy from the system.

Huh? What do you mean? Let's break down what each of those forces behave:

• Resistance (always -ve work): KE -> thermal/sound energy etc.

• Driving force (+ve work): chemical energy -> KE

• Weight (+ve work): GPE -> KE

• Weight (-ve work): KE -> GPE

Or if a diagram is more helpful (the blue shading represents the mechanical energy of the object):

Back to the question. If my hand do work on the object while weight converts that work to GPE, then it's totally logical that the mechanical energy of the object increases. The same amount of energy is transferred from chemical energy to KE of box, then from KE of box to GPE of the box. So nothing's wrong.

What about the car, then? From the beakers diagram above, the same energy is added to the system by the driving force and is removed from the system by frictional force. So yes, no energy change of the car.

Weight isn't like resistance. Weight doesn't take the energy out of the system, but rather transfers KE to GPE which is stored in the object.

And now the conundrum is solved, we can look at this problem again in terms of the work-energy principle. Does the box follow the rules of physics?

Well yes it does!