The kinetic energy comes from gravitational potential energy
The gravitational potential energy comes from the buoyant energy (force * elevation) lifting the balloon and brick into the air
The buoyant energy comes from the helium. So why should helium have buoyant energy?
Imagine taking a balloon full of normal air, and trying to push it underwater. That takes energy. Rather than gravitational potential energy, it is..... buoyant potential energy. The deeper you push the balloon of air into the water, the more potential energy you give the balloon of air.
The same goes for helium, except with the atmosphere. Helium has high potential energy near the ground, and expended potential energy high in the atmosphere.
The energy in the brick comes from the energy expended to collect helium and bring it to the ground. For example, some sources of helium that cost energy:
* Fusing hydrogen
* Mining out of the ground
* the geological processes that embedded hydrogen in the ground rather than the atmosphere in the first place
* Filtering/collecting out of the atmosphere
* and then the energy to pressurize and purify it into a compressed source of helium, which is used to fill the balloon
That is where the energy comes from!
Small tangent/elaboration:
* As mentioned in the comments, "Buoyant potential energy" (BPE) can be seen as another manifestation of gravitational potential energy (GPE). 621
If I were to elaborate this in as simple terms as possible, it would be the following: By pushing an air balloon into the sea (or helium down to the ground), the buoyant potential energy of the balloon (or helium) is = the gravitational potential energy of the water (or atmosphere) displaced. When you push a balloon of air into the ocean, you are one in the same displacing ocean water at a lower elevations to higher elevations. So if you reach the bottom of the ocean with your balloon of air, then there is a column of water above the balloon which you lifted against gravity (since the balloon displaces the water). The distance you lifted the column of water is the average height of the balloon. So the balloon's BPE = the displaced water's GPE. Then in the atmosphere, the helium's BPE = the atmosphere's GPE. It would be inaccurate to say the balloon itself has GPE, but it is indeed accurate to note GPE is involved. It is just the atmosphere, not the balloon that has GPE (if we are being very precise and technical when we speak about this). 326