The discussion revolves around the concept of gravitational potential energy (GPE) and how an object gains this energy when lifted. Participants explore the relationship between work done, potential energy, and the roles of external and conservative forces in this context.
Participants express differing views on the relationship between work, kinetic energy, and gravitational potential energy. There is no consensus on how these concepts interrelate, and the discussion remains unresolved.
Participants reference various principles, such as the work-energy theorem and the definitions of conservative and nonconservative forces, but the discussion does not resolve the implications of these principles on the concept of GPE.
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?