work energy theorem

[yoyohomieg5432]

does this apply to potential energy as well as kinetic energy? everywhere i've looked online it only seems to mention that doing work will change its kinetic energy. but it seems like if an object is suspended and i were to push it up to increase its potential energy, that involves me doing work that translates to potential energy

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[milski]

does this apply to potential energy as well as kinetic energy? everywhere i've looked online it only seems to mention that doing work will change its kinetic energy. but it seems like if an object is suspended and i were to push it up to increase its potential energy, that involves me doing work that translates to potential energy

Correct, it applies to kinetic energy only. Keep in mind that the work energy theorem applies only to the total work - that is the sum of the work done by all the forces on the object. Whatever work you did to change the potential energy by pushing the object up was matched with work with the same magnitude and opposite sign done by the conservative force related to the potential energy.

 

[yoyohomieg5432]

Correct, it applies to kinetic energy only. Keep in mind that the work energy theorem applies only to the total work - that is the sum of the work done by all the forces on the object. Whatever work you did to change the potential energy by pushing the object up was matched with work with the same magnitude and opposite sign done by the conservative force related to the potential energy.

i don't understand why a change in potential energy doesn't apply? If i raise an objects height it gains potential energy. I see what you're saying that gravity acts against it, but if the object is raised does its potential energy not increase?

 

[milski]

i don't understand why a change in potential energy doesn't apply? If i raise an objects height it gains potential energy. I see what you're saying that gravity acts against it, but if the object is raised does its potential energy not increase?

Sure, the potential energy increases. The work energy theorem does not make any claims about potential energy though. It relates total work to change in kinetic energy.

 

[yoyohomieg5432]

so if work and potential energy aren't related is it just coincidence that the work done on a charge in an electric field (qV) is the same as the potential energy of a charge in a field? (qEd=qV).

 

[milski]

so if work and potential energy aren't related is it just coincidence that the work done on a charge in an electric field (qV) is the same as the potential energy of a charge in a field? (qEd=qV).

You are missing a very important word there - only the total work is not related to the potential energy.

It is not a coincidence that the work done by the electric field on the charge is the change in its potential energy - that's more or less the definition of potential energy.

It is a coincidence that the total work done on a charge is the same as the change in potential energy. That can happen only if there are no other forces acting on the charge. In that case, the total work is the same as the work done by the E field. Since there are no other forces, at the end all of work done will be transformed in kinetic energy.

If any other forced was used to slow down the charge, that force would have done some work on the charge and the change of kinetic energy will be the sum of the work done by the two forces, or in other words it will be the same as the total work.