Can gravity be used as a source of energy? No, gravity cannot be used as a source of energy. Gravity can be used to convert stored potential energy into useful energy, but gravity in itself is only a way of storing energy. Gravity isn’t the stored energy. Gravity can provide a force that can be used to make energy, but to extract the energy from gravity something has to fall. And once it has fallen, you have to find a way to lift it back up again.
The simplest way to think about this is if we take a two liter bottle of Coca Cola and put it up on a shelf 1.5 meters high. That two liter bottle of soda weighs roughly 2 kg. To calculate the energy required by me to lift that bottle off the floor and put it on the shelf, we need to multiply the mass (2 kg) by gravitational acceleration (9.8 m/s squared) by the height of the shelf (1.5 m). If we do that, we get 29.4 joules. We know that energy cannot be destroyed, only changed. So, that 2 liter bottle of soda sitting on the shelf now has a gravitational potential energy of 29.4 joules. I produced that 29.4 joules by eating food and converting it into energy. The food I ate to get that energy either took it from the sun, or took it from plants that took it from the sun. If that bottle falls off the shelf, and there is no air resistance, it will hit the floor and that 29.4 joules will be converted into sound energy, heat energy, and other types of energy. If I used a million bottles of soda, I could use that energy to make electricity, but we can never get past the problem that I had to use energy to get the bottles of soda up there in the first place.
We already turn gravity potential energy into power on Earth. A hydroelectric power station might seem to get energy from gravity, but ultimately much of that energy came from the Sun. Solar energy evaporated water and lifted it into the atmosphere. Rain and snow then fell at higher elevations, giving the water gravitational potential energy. As the water flows downhill, we can recover some of that energy as electricity.
There are ways that this kind of system could be used to produce unlimited power, but they would only work in a perfect world. Take an electric car, for example. They can use regenerative braking. That means, when the car slows down, the brakes are used to regenerate power and recharge the battery. In theory, if a car goes up and down the same number of hills, it could regenerate enough power on the downhills to be able to go back up the uphills and never need to recharge. However, no system is ever 100% efficient and that doesn’t take into account things like friction and air resistance.
You can see the problem again if you think about a very long cable between the moon and the Earth. A mining company on the moon could break off giant bits of moon, fasten them to the cable, and then drop them towards the Earth. These pieces of moon would have an enormous amount of gravitational potential energy and they could be used to produce a staggering amount of power. However, after many many years, there would either be no moon left, or we would have to use power to get those pieces of moon that we had used back up there again.
One current way of thinking is to use gravity as a form of battery. A battery is just a way of storing energy chemically so that it can be reused at a later date. You charge the battery, adding in the power. The battery stores it, and when you want the power you can take it back out. You can do the same with gravity. Some natural energy companies, such as solar and wind, are using gravity as a way to store the power. The problem with renewable energy is that at some times of the year you can produce more than you need and at other times you can’t produce enough. What one company is doing is that when they produce too much power, they use the excess power to lift an enormous weight up a mine shaft. The weight is about the size of a bus and weighs 700 tons. When they reach a time of year when the renewable energy can’t produce enough power, they lower the weight back down, turning a generator, and changing the gravitational potential energy back into electricity. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Gravity_battery
https://www.mane.co.uk/resources/blog/can-gravity-be-used-as-an-energy-source
Photo by Rafael Minguet Delgado: https://www.pexels.com/photo/colorful-water-droplet-with-red-and-blue-background-39415739/