GPE: How Does an Object Gain Potential Energy?

[UMath1]

Gravitational potential energy is equal to negative work. But in the case of lifting an object upwards, the work done on the object would be 0. The work on the object by the lifter would equal the work on the object by gravity. Then, how does the object get GPE?

 

[Doc Al]

Gravitational potential energy is equal to negative work.

The change in GPE equals the negative of the work done by gravity.

But in the case of lifting an object upwards, the work done on the object would be 0.

Work done by who? To lift the object the lifter must do work on it.

The work on the object by the lifter would equal the work on the object by gravity. Then, how does the object get GPE?

True, the net work is zero, but that only means that there is no change in kinetic energy.

The object gains GPE because someone did the work to lift it.

 

[UMath1]

Isn't it true however that -Delta K=Delta U? Where U is potential energy, and K is kinetic energy. It is also true that Wnet= Delta K. So if net work is zero, then change in kinetic energy should be zero, and therefore change in gravitationak potential should be zero as well...?

 

[Doc Al]

Isn't it true however that -Delta K=Delta U? Where U is potential energy, and K is kinetic energy.

Not if there's an external force (the lifter) doing work on the object.

It is also true that Wnet= Delta K.

Yes, that's the work energy theorem.

So if net work is zero, then change in kinetic energy should be zero, and therefore change in gravitationak potential should be zero as well...?

Nope.

 

[UMath1]

Why does the lifter count as external but gravity does not?

Is it because the lifter applies a nonconservative force?

 

[Doc Al]

Why does the lifter count as external but gravity does not?

Realize that GPE already accounts for the work done by gravity. So if you use GPE you do not also treat gravity as doing work--to do so would be to count it twice.

 

[Doc Al]

Is it because the lifter applies a nonconservative force?

You can think of it that way. The conservative force of gravity is already included in the GPE; the lifter's non-conservative force is not.

 

[Dale]

Is it because the lifter applies a nonconservative force?

It is possible that the lifter will be a conservative force, like a spring. Then the lifter will have its own potential energy which will be different from GPE, like elastic potential energy.