# Is work a transfer of energy in general, or is it a transfer of kinetic energy?

the net-work is the change in kinetic energy, so is every one of the "works" done on an object solely transferring energy through kinetic energy?

In other words, if I was to lift an object the object would gain potential energy because of the nonconservative work done by my hand, but given that gravity does work as well (though through conservative forces) it does influence the work-energy equation and balances the kinetic energy to be 0 (if both of the "works" done are equal).

In that situation is the object gaining kinetic energy through work, which is then stored as potential energy by the work done by gravity? or is the work just directly transferring its energy as potential energy?

I'd recommend applying Occam's razor: Don't multiply entities unless strictly necessary. Holding that a body moving at a steady speed is losing kinetic energy as fast as it is gaining it has, in my opinion, no explanatory power. Just leave out KE from your energy narrative (except for the small gain in KE at the start of the motion, and loss of KE at the end).

In other words, if I was to lift an object the object would gain potential energy because of the nonconservative work done by my hand, but given that gravity does work as well (though through conservative forces) it does influence the work-energy equation and balances the kinetic energy to be 0 (if both of the "works" done are equal).

Gulp. That sounds rather confused.

An object of mass $$m$$ is increased in height from $$h_1$$ to $$h_2$$. The work done on the object is:

$$\text{d}W=F(h)\text{d}h=mg\text{d}h$$

$$W=\int_0^W\text{d}W=\int_{h_1}^{h_2}mg\text{d}h=mg(h_2-h_1)$$

This is of course also the increase in potential energy $$\Delta U$$, so $$W=\Delta U$$. KE, quite literally doesn't enter the equation.

the net-work is the change in kinetic energy,[...]

In some cases (like accelerating or braking) yes, but obviously not in all.

• But if you were to analyze the motion not just by the endpoints, you would find that there is a net work at the beginning that increases the kinetic energy, and then a net work at the end that decreases the kinetic energy. I think this is what the OP is focusing on. Also, you should specify that $W$ is not the work done on the object (overall); it is the work done on the object by a force of magnitude $mg$. Commented Jan 21, 2021 at 22:51
• @BioPhysicist I don't see the point in NOT focusing on the end-points, unless for very specific purposes that haven't been specified.
– Gert
Commented Jan 21, 2021 at 22:56

the net-work is the change in kinetic energy, so is every one of the "works" done on an object solely transferring energy through kinetic energy?

No. Work can change both potential and kinetic energy, but net work only changes kinetic energy. See below.

In other words, if I was to lift an object the object would gain potential energy because of the nonconservative work done by my hand, but given that gravity does work as well (though through conservative forces) it does influence the work-energy equation and balances the kinetic energy to be 0 (if both of the "works" done are equal)

Having a bit of difficulty following you.

But gravity does not effect the work energy theorem because when you lift an object from rest and bring it to rest at a height $$h$$ you do positive work of $$mgh$$ while at the same time gravity, because its force is opposite the displacement, does an equal amount of negative work of $$-mgh$$ for a net work of zero. In effect, gravity takes the positive work you do and stores it as gravitational potential energy of the earth/object system.

Since in this example the object begins and ends at rest, the change in kinetic energy is zero. But the work you did is stored as gravitational potential energy.

In that situation is the object gaining kinetic energy through work, which is then stored as potential energy by the work done by gravity? or is the work just directly transferring its energy as potential energy?

It is not gaining kinetic energy through work if the object begins and ends at rest, though it is gaining gravitational potential energy, as I explained previously. If it is not brought to rest at $$h$$, then the object will have both KE and gravitational PE, but only the KE is due to net work.

Hope this helps.

Potential energy is just another way of expressing the work done by a conservative force. Potential energy is just a convenient way of expressing the work done by a conservative force.

In mechanics, work is the change in kinetic energy due to the net external force acting through a distance.

Work in thermodynamics is a broader concept, meaning: energy that is transfered, without mass transfer, across the boundary of a system, because of any intensive property difference other than temperature between the system and its surroundings. Heat is energy that is transfered, without mass transfer, across the boundary of a system, solely due to a temperature difference between the system and its surroundings.