eli5: If space is a vacuum, how can rockets work? What are the thrusters pushing *against* if there is nothing out there?

Question

I’ve never really understood the physics of this. Obviously it works somehow — I’m not a moonlanding denier or anything — but my (admittedly primitive) brain continues to insist that a rocket thruster needs something to push *against* in order to work.

So what is it pushing against if space is essentially a void?

In: 7157

Answer 1

Rockets don’t work by pushing off anything. They work by throwing stuff out the back.

It is a law of physics that every force has an equal and opposite reaction (Newton’s third law of motion). This basically means if you push on something, it also pushes back. If you were sitting in a rolling chair and threw a bowling ball, you’d roll in the opposite direction you threw the ball. This works with anything that has mass.

The hot gasses you get from burning fuel have mass. Throwing them out the back of the rocket puts a forward force on the rocket. The faster you thrown the gasses out the back, the greater the force on the rocket and the faster it goes.