Gravitational potential energy (GPE) is defined as the energy an object possesses due to its position in a gravitational field, and it is equal to the negative work done by gravity. When an object is lifted, the work done by the lifter is non-conservative, allowing the object to gain GPE despite the net work being zero. The work-energy theorem states that net work equals the change in kinetic energy, but in this scenario, the external force (the lifter) contributes to the increase in GPE without affecting kinetic energy. Thus, GPE accounts for the work done by the lifter, while gravity's work is already factored into the GPE calculation.
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The change in GPE equals the negative of the work done by gravity.UMath1 said:Gravitational potential energy is equal to negative work.
Work done by who? To lift the object the lifter must do work on it.UMath1 said:But in the case of lifting an object upwards, the work done on the object would be 0.
True, the net work is zero, but that only means that there is no change in kinetic energy.UMath1 said:The work on the object by the lifter would equal the work on the object by gravity. Then, how does the object get GPE?
Not if there's an external force (the lifter) doing work on the object.UMath1 said:Isn't it true however that -Delta K=Delta U? Where U is potential energy, and K is kinetic energy.
Yes, that's the work energy theorem.UMath1 said:It is also true that Wnet= Delta K.
Nope.UMath1 said: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...?
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.UMath1 said:Why does the lifter count as external but gravity does not?
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.UMath1 said: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.UMath1 said:Is it because the lifter applies a nonconservative force?