Is gravity endless energy?

Question

You need energy to produce a force that compensates gravity. So if you apply that force for a huge amount of time, will gravity ever be depleted?

In: 39

Answer 1

The full explanation is a bit complex, but I will try to simplify it a bit:

I am assuming that you mean that we could, hypothetically, extract energy out of Earth’s gravity. To do this, you need an object above the surface of earth in a high place.

Let’s say you have a baseball that is placed one meter above the ground. This baseball has more gravitational energy than if it were on the ground. However, in order to extract that energy, you need to make the baseball fall down from its high place and onto the floor, and while it falls down, you need to transfer some of that energy into a motor (by colliding the ball onto a paddle that moves the motor, for example), and then transform the movement of the motor into something else (technical stuff that is not important right now). But once the baseball reaches its minimum height (the ground), then all the “gravitational energy” is already depleted, and is usually dissipated onto something else (transformed into heat, usually).

This is the basic principle of hydroelectric power plants, because water can be see as “a bunch of baseballs” that are falling down from a high place following a stream due to Earth’s gravity, and we simply use motors attached to paddles that collect a little piece of that energy.

However, in order to collect that energy, you need an object falling down from a high place (water/baseballs), and in order to get those objects back up, you need to invest energy in carrying them above the ground (you need to put the baseball on top of the shelf for it to be able to fall down), and that usually requires more energy than what you usually can harvest when if falls back down. In the case of water stream, what makes the water “rise above” is its evaporation because of the heat of the sun (and the water cycle, that later makes it rain). So in a sense, hydroelectric power is just gravitational energy harvesting powered by the sun.

So, to summarize, gravitational energy is associated with the height of an object, not the force that pulls it downwards, and it can only be harvested when the object moves downwards, so it is impossible to obtain unlimited energy from gravity.