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I understand that when you drop an object, the potential energy will convert into kinetic energy which will then be converted into vibrations of the air and ground when it hits the ground.

But what about when an object is being lowered at a constant speed? Here, the potential energy would be decreasing constantly, but the kinteic energy would remain the same. Where does the energy go in this case?

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To be lowered at a constant velocity there must be a force on the falling object of mass $m$ equal in magnitude but opposite in direction to the force on the object due to the gravitational attraction of the Earth $mg$.

In falling a distance $\Delta x$ the falling object does work $mg\,\Delta x$ on whatever is providing the upward force.

Whatever is providing the upward force could be the air when the object is falling with a terminal speed, it could be an electrical generator which is attached to the falling object via a piece of string, etc.

Put another way the object (and the Earth) are losing gravitational potential energy and that is being converted into another form of energy dependent on what is providing the upward force.
If it is air with the object falling at a terminal velocity it would be heat.
If it is a generator then it is electrical energy.

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