We cannot use gravity as an infinite energy source. This is because gravity cannot actually be used as an energy source at all. Since gravity is a force, it is not an energy source.
Energy and force are two different things and should not be confused. Where energy can be described as being a property in an object, a force is the description of the interactions that occur between them.
Energy can be in atoms, light beams (photons) and even batteries, but forces are the way that this energy is transferred between two objects when they interact. Therefore, forces are not themselves a form of energy. As a force, gravity produces a way for objects to transform energy into other states.
This can be a little difficult to wrap your head around, so imagine this. You are holding something heavy like a bowling ball and you are stood at the top of a big hill. If you were to let go of the bowling ball it would fall down the hill and seem to gain some level of momentum.
It may seem that gravity is giving energy to the bowling ball on its journey down the hill, but actually, gravity is just a force. The energy that the bowling ball is exhibiting as falling and rolling actually came from your arms and muscles as you carried it to the top of the hill and let go.
As a force, gravity allows for the temporary storage of energy in an object. The energy that the bowling ball gained with being carried to the top of the hill is called potential energy.
This potential energy is the energy that an object gains when it is being lifted against gravity, like you carrying it up a hill. Therefore, the force, gravity, allows for the energy to be transferred from the lifting agent, your muscles, to the other object, in this case the bowling ball.
Gravity will convert the potential energy of the ball into kinetic energy, hence the ball falling and moving. Interestingly, in keeping with the balance, the bowling ball cannot gain any more kinetic energy than it already had potential energy from you lifting it up the hill.
So, the energy that it exhibits in the form of kinetic energy is fully dependent on the amount of potential energy that it accumulated when it was carried up the hill. Gravity will not affect this, only facilitate the exchange of energy.
Interestingly, this concept is true of more forces than just gravity. The fact that a force will facilitate an exchange of energy. A clearer way to envisage this idea is to imagine two strong magnets that you are pulling apart with all of your strength. When you let go of the two magnets, they will appear to hurtle towards one another and gain speed.
This is not because of the magnetic force; it is actually because you pulled them apart from each other against that magnetic force. The potential energy that was stored in the magnets thanks to you pulling them apart is simply exchanged into kinetic energy by the magnetic force. This also proves that when you are pulling an object away from a force, you are always giving it potential energy.
Gravity does not suddenly go away or leave a scenario. In this sense, it is somewhat unlimited in that it cannot ‘turn off’. This is true of an object as long as it has mass that gravity can act on. But gravity is a force, so the nature of its ‘unlimited’ source cannot be syphoned or extracted to use as energy.
Similar to the example with the magnets, imagine a rubber band. You can stretch a rubber band to its limit, and this will store potential energy in it. When you let go of the rubber band, this potential energy become kinetic energy, causing the band to spring back to its usual shape.
A rubber band that is sat still on a table will not move at all. If you were to pick up this same rubber band and stretch it until it snapped, this would be because of you stretching it, and not because of the rubber band itself. Lifting an object from the ground against gravity is just like stretching that rubber band. Both cause a build-up of potential energy, and both have gravity facilitate the exchange of energy into kinetic energy.
The negative thing about confusing energy with forces is that it leads to things being created like free energy machines. These fail because forces are simply not energy and you cannot extract energy from a force. A free energy machine could rely on the same bowling ball falling down a hill and turning a wheel at the bottom.
Although this appears to be ‘free’ energy, what happens when you need to more and the bowling ball has to somehow get to the top of the hill again? And you also need to consider that some energy is lost to heat, so you are never getting the exact same energy that is inputted.
As well as the fact that the energy it would take to take the ball back to the top of the hill would equal the energy you get out from the turning wheel.
So, what is the point? The point is that free energy is not a real thing. The reality is that you can never get more energy out of this system than you put in. Meaning that the concept of free energy and free energy machines is a categorically flawed one.
So, you might be asking about a place like a hydroelectric plant that extracts energy from falling rivers. Are they extracting energy from gravity? The answer is no. The reason being that the water falling is the exact same as the bowling ball rolling down the hill. The water that falls has gained its energy from being high up in the first place, just like the bowling ball got its potential energy from being carried up the hill.
Interestingly, the main provider of energy in this case is the sun. sunlight will warm the ocean, cause evaporation and the water to float in the sky. This is where it’s a little more complex: the energy in the photons of light is converted to the potential energy of the water molecules that have just been evaporated.
As we all know, this evaporated water then falls down to the ground as rain, creates rivers and eventually gets to our hydroelectric plant in the form of a waterfall. This potential energy is converted to kinetic energy in the process. And then further exchanged to electric energy by the plant. All of this means that hydroelectric water plants are actually converting solar energy, as that is what caused the water to evaporate and start its journey, into electricity from water.
To summarise: gravity can be classed as unlimited, but that does not make it a source of energy. Gravity is a force and can work as a facilitator to exchange one type of energy into another when a potential energy is achieved in another way.
