How do we calculate the work done by gravity?

[Monkeymaniac]

W (work done on an object) = dKE + dPE.

When a person lifts up (non-conservative force) an object from ground to hegiht h, he's changing the potential energy of the object to mgh. So the work done on the object by the person is mgh. Obviously, gravity points in the opposite direction from the object's displacement, so the work done by the gravity on the object in this case would be -mgh.

Then, if we are asked what's the net work done on the object, do we say that it's zero (mgh + -mgh)? It seems that this would then conflict with the equation above, which predicts that W would be a non-zero value (due to non-zero dPE). What's going on here? Does W above only refer to work done by non-conservative forces?

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[Monkeymaniac]

Anyone please?

 

[fizzgig]

i remember this being in EK.

if an object already had velocity and shot upwards, the conservative force of gravity would be doing the slowing, and when it stopped at its peak the E would be stored as PE so conservative force, no work done.

if an outsider (animal, person, etc) does the lifting and you want to do conservation of energy, then you count the work of the outsider only.

from your equation dKE is 0 (started at 0, accel to some velocity while being lifted by a person, then back at 0 once it's set on a ledge or whatever). dPE has increased -- so work was done on the object, against gravity, and the object now has more energy than it did. you increased PE without spending KE because the person lifting provided the E.

 

[davesmith]

Hello,

There are many different formula for calculating the work done by gravity.

Thanks,
Dave Smith

 

[ru4real1666]

This is stupid.

almost always, W done by gravity = - dPE