Eli5 how decreasing the diameter of the nozzle (of a rocket for example) makes the rocket go faster? It’s still the same amount of gas exiting the rocket no matter the diameter, so why is there a difference?
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Because it’s not just the mass of gas that matters, it’s the momentum (mass * velocity) of the gas that affects the momentum (mass * velocity) of the rocket.
Look at it this way: guns have recoil, right? Do you get the same recoil if you just hold a bullet and “let go” of it? Same mass, completely different bullet speeds between shooting it from a gun and just letting it drop out of your hand. And the “recoil” is what makes the rocket go up.
Decreasing the diameter of the nozzle forces the gas to exit faster; it’s the same principle as squeezing a water hose to make it shoot water further away.
Note that for supersonic speeds the nozzles are shaped like “widening” bells because *increasing* the diameter makes the gas go faster, not decreasing it.
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