system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can gravity exert a force without expending energy?

Every other thing in the world, if you want to accelerate it, you need to spend energy. But not gravity, which pulls distant objects like meteors down without spending any. Doesn’t this violate the 2nd law of thermodynamics or something?

EDIT: Thanks for all the great answers and discussions! Apologies in advance if I can't reply to them all.
gpt
> Every other thing in the world, if you want to accelerate it, you need to spend energy.

This is not true. Something with a positive electric charge will accelerate toward something with a negative electric charge. No energy input required. Acceleration does not require energy. Kinetic energy is a function of *velocity*, not acceleration.

People here talking about how gravity is a special force (it is) are missing the point. Even from a Newtonian understanding, nothing is being violated.

Think of it this way. You can’t get free energy from gravity. Sure, gravity can make an object go fast… as long as you first spend energy to lift the object off the ground. The kinetic energy all comes from the gravitational potential energy that the object started with.

Edit: to clarify, no energy *input* to the system is *required* to have a constant acceleration. An object accelerating in a field by changing speed (not just direction) does gain kinetic energy… but loses potential energy. Total energy is the same. You *can* spend energy to get acceleration (like burning fuel in a car), but forces like gravity and electromagnetic force can produce acceleration just from an object being in the field.

I guess it might be clearer to say that objects in the field always had potential energy to begin with, so if the force causes the object to lose potential energy, it gains kinetic energy. But the force can also make an object “lose” energy, for example, a comet that was already moving at high speed *away* from a planet gets slower as the planet’s gravity pulls on it. It accelerates, but *loses* kinetic energy (by gaining potential).