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I would say "this isn't rocket science" but for the low-level of flat-earthism, perhaps rocket science is actually necessary! HAHAHA
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A rocket engine doesn't need to "push off" against anything to work.
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You can see this is the case with a toy balloon -- just blow it up and let it go, and it accelerates AWAY from where the air comes out.
In the case of the balloon, the wobbling air nozzle changes direction so the balloon goes to and fro not in a straight line.
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A rocket engine can operate just fine in a vacuum where there is no ambient air.
It doesn't need any air to "push off from" because there is a force inherent in the velocity and mass of the escaping gasses.
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You have to figure in the weight (mass actually) of the gasses escaping AND their speed (or velocity) because force is a product of these two very different things.
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That's why when flat-earthers try to explain something by saying the weight exceeds the velocity you can tell they don't know what they're talking about.
Because weight cannot exceed velocity. Velocity cannot exceed weight.
Weight and velocity are like apples and oranges. You can't compare them to each other.
Unless, that is, you enjoy mixing apples and oranges.
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(https://s14-eu5.ixquick.com/cgi-bin/serveimage?url=http%3A%2F%2Fdocxmd.com%2Fwp-content%2Fuploads%2F2016%2F07%2FApple-not-equal-to-orange.jpg&sp=399c32dbf835e0e5e44afb96eab0dda0)