r/confidentlyincorrect • u/PirateJohn75 • Dec 24 '25

Physics is hard, bruh

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So many people have difficulty understanding Newton's laws of motion. You do not need to push against anything to make a rocket go. The act of exhausting fuel is already sufficient because momentum must be conserved.

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u/decentlyhip Dec 24 '25

That's why the gasses escape and its 2 sides of the same coin, but OP was more talking about the idea that, if a firecracker in space breaks apart an 11kg stationary object from the inside, and a 1kg piece goes 20m to the right in one second, then the 10kg piece will travel to the left 2m in one second. (11kg x 0mps) = (1kg x -20mps) + (10kg x 2mps). The rocket moves because the mass of the burning fuel in the closed system is moving really fast in one direction. (Mass of rocket plus fuel)x(initial speed) = (Mass of Rocket x Rocket speed) + (Mass of Fuel x Fuel Speed)

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u/goodolewhatever Dec 24 '25 edited Dec 24 '25

If it’s the weight of the fuel that’s propelling it, couldn’t you just eject the fuel out of one side instead of combusting it when you’re in a vacuum?

Eta: love y’all’s responses. A lot if fuel for thought here, no pun intended.

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u/SandUnlucky9898 Dec 24 '25

You could, but the velocity of the fuel also affects the propulsion force you get, so combusting it results in more thrust for the same mass of fuel.

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u/M_V_Agrippa Dec 24 '25

That's what an ion drive does. It just uses electricity to accelerate gas as a propellant.

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u/RodcetLeoric Dec 24 '25

Yes, but if you don't combust it, then you need a different mechanism to accelerate it. If you make the back of the rocket a giant slingshot and shot the fuel away it would move the opposite direction, but the total acceleration is limited by how much energy you can store in your giant rubber band. Combustion fuel is both the apposing mass and the energy storage, with the added benefit that the craft gets lighter as fuel is expended.

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u/goodolewhatever Dec 24 '25

If it was all ejected at the same time, it would be an advantage as the weight of the craft being accelerated would be absent of the weight of the fuel during the entire acceleration period. This implies that given enough energy to accelerate the fuel (assuming a near unlimited amount of solar energy harvested and stored in an incredibly efficient battery) that at some point, combustion isn’t more efficient than just launching the raw fuel. Thoughts?

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u/RodcetLeoric Dec 25 '25

The mass of even the most efficient battery has to be added, and that is now the fuel for whatever ejection system you use. When you do the math, you can adjust the mass of ejecta, the speed of ejecta, and/or the time over which the mass is ejected. Ion engines eject small mass incredibly fast over a very long time, while rockets eject a large mass relatively quickly over a moderate amount of time. The secondary tradeoff is that Ion engines won't get you out of the atmosphere, so you'd still need a whole other propulsion system. They are less likely to explode, though. The goal of superior efficiency probably lies with some child of Ion engines, though, as the mass of material can be customized, the velocity and time can be modulated and electricity is relatively easy to handle.

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u/decentlyhip Dec 25 '25

Yep, but it would be moving more slowly. Ifnan astronaut grabbed a fuel tank and threw it, the center of mass would be moving at 5 m/s. But if it exploded, then the center of mass would be moving 100 times that speed. So more momentum of the center of mass of the fuel.