conservative energy

[airbornemedic]

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I've been studying out of exam crackers physics 7th edition. In the portion on work and energy it states that there is no work done by conservative forces because the change in kinetic energy plus the change in potential energy add up to zero, also conservative forces cannot change internal energy.

This all makes sense, but this technicality would not work well for the MCATs right? If I was asked how much work gravity did, sure it's a conservative force so as kinetic energy increases, potential energy decrases and there is no change in total energy. However, I would still have to answer the work done is the force applied (mg) over the distance which the object has fallened right?

Another statement I don't understand is "The work done against conservative forces is conserved In potential energy". Say for example the falling object was stopped by me and I lift it to a greater hight. In this case I'm doing work against a conservative force, and the poitental energy is not conserved, gravitational poitental energy is increasing.

any help woud be great

 

[sleepy425]

A conservative force will do 0 work in a closed path. This is very important, because gravity, which is a conservative force, does work to move an object from 10 m to 5 meters. But if you move an object from 10 m to 5 m and then back to 10 m, no work is done by gravity. In addition, if you move an object from 10 m to 5 m, and then back to 7 m, gravity did the same amount of work as if it moved the object directly from 10 m to 7 m.

Internal energy is a separate matter. Assuming no other forces are acting on the object (just gravity), then yes, internal energy will not change since, as the object falls, its potential energy becomes kinetic energy, so the sum is always the same.

As for your second question, I just figured out what you were asking. When you say, "conserved IN potential energy," that is not saying that potential energy is conserved. It simply means that the energy exerted to move the object (to do the work against the conservative force) doesn't dissipate, it gets conserved (stored) in the form of gravitational potential energy.