# Space.

A thought just entered my head!... If Space is a void where objects travel for infinity etc, how do rocket thrusters have any use?
Surely to make a rocket move with a thruster it has to have a force to thrust against?
Also would a bird be able to fly in Space as there`s no energy or force for it`s wings to react against?
10:38 Thu 16th Jul 2009

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 This is a common misconception. (In fact, I think the New York Times ran an article about a British rocket enthusiast's dreams of using a rocket in space (back in the 1920's or so), saying that it wouldn't work for this very reason.) 1) Space is not a void. There's all kinds of plasma and other crap out there. This is not including the forces, like gravity, and electromagnetic radiation. 2) Use Newton's second law to understand. It says F=ma, which means that the force of an object is equal to its mass multiplied by its acceleration. Another way to look at this is that force is equal to the rate of change of the object's momentum. Momentum, mass multiplied by velocity, includes a mass term. As mass is displaced, Newton's third law (equal and opposite reaction) assures a force in the opposite direction, because there is a change of momentum (thus, a rate of change of momentum). So all you have to do in an ultra-low friction environment, such as between the Earth and the Moon, is kick out some stuff from the back. What the Saturn V actually did (well, part of it), is power the astronauts out of the Earth's atmosphere, and then stop. They had enough momentum to just keep going (Newton's first law). 11:48 Thu 16th Jul 2009 A bird would not be able to fly in space, because it isn't kicking out any mass. It relies on pushing air with its wings. 11:49 Thu 16th Jul 2009 -- answer removed -- -- answer removed -- I wonder if I can put it less technically (not my forte but oh well) Imagine you're firing a shotgun - the recoil kicks you backwards. That's not dependant on there being air. The shot is propelled forward by pushing off of you - and that pushes you back. As Fo3nix says this is technically momentum (mass times velocity) conservation. If you are 100Kg and throw a 10 Kg mass forward at 1 m/s You are thrown back at 0.1 m/s That is because 10kg x1ms = 10 = 100kg x 0.1 m/s So the momentum in one direction balances that in the other. What stops you moving in the shotgun example is the friction on the soles of your shoes. If you were on ice skates you'd go back 11:57 Thu 16th Jul 2009 Jake: that's a good example. I'd thought about an ice skater blowing out birthday candles, but this still seems to assume air (in some naive sense). The shotgun analogy is much better. 12:43 Thu 16th Jul 2009 This came up a few weeks ago, Newton's first law dear boy! 19:17 Thu 16th Jul 2009 My science teacher explained it as like an astronaut throwing a bowling ball in space. He never was any good at analogies though :) 19:20 Thu 16th Jul 2009 Question Author Brilliant! fo3nix and jake-the-peg many thanks for that. If any other thoughts enter my head "I`ll be back" 19:26 Thu 16th Jul 2009 For a rocket air is a drag 19:36 Thu 16th Jul 2009

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