When is Gravitational Potential Energy Considered in the Work-Energy Theorem?

[Generally Confused]

When using the work-energy theorem (Wnet=ΔE), when do you take gravitational potential energy into account? Change in energy implies all types of energy involved, but in what cases would PEg be a part of it?

 

[PhanthomJay]

That is not a correct statement of the work-energy theorem. It states that the net work is the change in the kinetic energy of the system, where the net work (total work) includes the work done by both non conservative forces (like friction or applied contact forces) and conservative forces (like gravity and spring forces). The latter encompasses the potential energy change of the system , if any. Another way to look at this is to use the conservation of energy principle where by the work done by non conservative forces is the change in kinetic and potential energies. You should compare the two and conclude they are the same. The total energy change of the system, when you include heat and other forms of energy generated by the work done by non conservative forces, must be zero, since energy cannot be created or destroyed.

 

[Hydrous Caperilla]

When using the work-energy theorem (Wnet=ΔE), when do you take gravitational potential energy into account? Change in energy implies all types of energy involved, but in what cases would PEg be a part of it?

Work energy theorem considers conservative and non conservative forces into consideration..The net work done by all these forces are to taken into consideration when applying these theorem.So everytime you are using Work Energy theorem ,you are consciously or uncosciously consiering Work done by Gravity though it's another matter that it can be zero.

 

[jtbell]

There are two kinds of forces: conservative forces, which have potential energy associated with them, and non-conservative forces, which don't.

There are also two versions of the work-energy theorem. The first one, W = ΔK, says that the net work done by all forces (both conservative and non-conservative) on an object equals the change in the object's kinetic energy. Potential energy isn't mentioned here at all.

The second version, W_nc = ΔE = ΔK + ΔU, says that the net work done by all non-conservative forces equals the change in the object's mechanical energy (kinetic plus potential). In effect, the work done by the conservative forces has been moved over to the other side of the equation and relabeled as the change in potential energy.

 

[Generally Confused]

When using the work-energy theorem (Wnet=ΔE), when do you take gravitational potential energy into account? Change in energy implies all types of energy involved, but in what cases would PEg be a part of it?

This is word-for-word what we are learning in my beginner´s physics class. Although it may not exactly be correct, does anyone have an answer under these circumstances? This all we learned on the topic at this point.

 

[jtbell]

If you're using the first version of the work-energy theorem, you don't use gravitational potential energy at all; instead, you include the gravitational force in calculating W_net. If you're using the second version of the work-energy theorem, you don't include the gravitational force in calculating W_net (which I labeled W_nc in my other post); instead, you include the gravitational potential energy on the right-hand side as part of E.