# Lifting weights - gravitational potential energy

If I lift a weight to a certain height, where does the gravitational potential energy at that height originate from?

is kinetic energy converted into gravitational potential energy as the weight is decelarating as it approaches the maximum height? or is the chemical energy from my body (doing the work) shared between kinetic energy and gravitational potential energy?

Hope this makes sense. thanks.

Initially when you push an object from rest to a certain height, you do have to make the object initially accelerate so some of that energy is used towards kinetic energy atleast momentarily and the normal force(applied force) is temporarily greater than the gravitational force down. However after that, whether the energy is used for more kinetic energy and gravitational potential energy, or just gravitational potential energy depends on whether the block is accelerating up or going at a constant velocity up.

If the block is accelerating up, then it means that you are applying net work on the object because there is a force up and a displacement up, so the kinetic energy increases and the potential increases because some of the energy is used to place the block higher up.

If the block is going at a constant velocity, the energy provided goes into increasing just the height.

So basically the chemical energy turns into potential energy always, but can additionally turn into kinetic energy if the block is accelerating up.

• But don’t throw the weights for normal training. Smooth and easy.
– JEB
Commented Jun 4, 2023 at 16:43
• so if the object is going up at constant velocity, the kinetic energy generated from its movement is immediately converted to potential energy? From what I understood, any object at constant velocity maintains its KE? Commented Jun 4, 2023 at 16:48
• If an object moves up at a constant speed, its kinetic energy is constant and there is no kinetic energy that is gained anymore. The chemical energy then directly converts to potential energy. Commented Jun 4, 2023 at 19:26

If I lift a weight to a certain height, where does the gravitational potential energy at that height originate from?

It originates from the chemical potential energy stored in the cells of your body. That energy is converted to kinetic energy when you perform positive work on the weight when you lift it. At the same time you do positive work on the weight transferring energy to it, gravity is doing negative work on the weight taking energy away from it (the work of gravity is negative because the force of gravity is in the opposite direction of the displacement of the weight).

If the weight begins at rest before you lift it and ends at rest at a height of $$h$$ then its overall change in kinetic energy is zero. From the work energy theorem that means the positive work you do on the weight of $$mgh$$ equals the negative work done by gravity of $$-mgh$$, for a net work of zero. This occurs if initially your upward force is greater than gravity accelerating the weight giving it kinetic energy. Then prior to reaching the final height you reduce your force to less than gravity decelerating the weight taking away the initial kinetic energy bringing it to rest.

Finally, the energy $$mgh$$ taken from the weight by the negative work done by gravity is stored as gravitational potential energy of $$mgh$$ in the earth-weight system.

Hope this helps.

• "gravity is doing negative work on the weight taking energy away from it" why is gravity doing negative work? the gravitational force on the weight is not causing a transfer of energy? Isn't the only form of work being done in this situation related to the conversion of chemical energy to kinetic energy whilst the weight is moving at constant velocity? Commented Jun 5, 2023 at 3:07
• @mar01 What is the mathematical definition of work? Commented Jun 5, 2023 at 11:35
• ahh makes sense. force x displacement Commented Jun 5, 2023 at 12:10
• @mar01 Yes, and negative work on something transfers energy away from something instead of to something. Commented Jun 5, 2023 at 12:34