system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Emdrive. Why does this violate Newton's third law and how does it work?
gpt
Conventional rockets work on Newton's Third Law, or simplified: you can't push yourself forward without pushing something else back. When you jump in the air, you're pushing the Earth away with the same amount of force that's launching you into the air. But because the Earth is so many millions of times heavier than you, the amount it moves away from you is immeasurably small. A rocket pushes itself forward by pushing exhaust out from behind it. Each individual gas particle is small, but all of them moving together very quickly in the same general direction are enough to push the rocket the other direction.

the EMDrive supposedly works by generating a resonance of microwaves inside a conical chamber. Supposedly, and without getting into graduate-level physics, the resonance is stronger at one end of the cone because it's concentrated down, and the sum of the forces is a very very small but nonzero value. In effect, that would mean that they are producing force without expending exhaust mass.

It's important you know that the EMDrive almost certainly doesn't work. The force that was measured was extremely small, and it is far more likely that some other variable was generating a phantom force than that the engine is working as intended.