Since a spaceship is extremely heavy, a great deal of material must be moved downwards. To do that, something else must be moved downward. “Now imagine a spaceship that weighs a hundred thousand tons lifting off Earth. It's the old principle of ‘You can't get something for nothing.' “Nothing can move forward unless something else moves backward. When we fly a gyro-flivver, we push air backward and move forward.
When we walk, we push back against the ground and move forward. When we swim, we push water backward and move forward ourselves. Sometimes they call it Conservation of Momentum. Sometimes they call it Newton's Third Law. They call it the Law of Action and Reaction. Even Rioz felt a slight tingling of suspense, though he had spent the greater part of his life aboard ship. He made spaceship propulsion sound like the secret of the ages, like an ultimate revelation. What moves it? What gets it off the Earth?”Įveryone knew what moved a spaceship, but Hilder's voice was like a drug. “But this is only the travel-head of the ship. The communications system of the ship outlined itself in red as he talked about it, the storerooms, the proton micropile drive, the cybernetic circuits… Hilder's voice continued, pointing out features of interest that appeared in schematic color. He's giving the whole works-color cartoons, trick photography, stills from old films, everything.”Īs if to illustrate Long's remarks, the bearded figure faded out of view, and a cross-sectional view of a spaceship flitted onto the screen. He said, “What's the Grounder been talking about?” Space was clear, so to hell with sitting and looking at a blank, green, pipless line. But Hilder needs an issue if he is going to be elected as global co-ordinator) Rioz took a hesitant step forward. Hilder is making a fake controversy about Mars wasting water as rocket propellant, which is nonsense. It is a propaganda piece from a nastly little politician named "Hilder". They are listening to a broadcast from Terra. (ed note: Long and Rioz are Scavengers: citizens of Mars whose job is to intercept cast-off propellant tanks, brand them, and send them to Deimos. The practical effect is even if the mass of the propellant shooting out the engine is tiny compared to the spaceship, if the propellant is moving really fast the recoil will give the heavy space ship a substantial velocity in the other direction. Which is a fancy way of saying that an object with a tiny mass but an enormous velocity can have the same momentum as an object with a huge mass but a tiny velocity. Momentum is the object's mass times the velocity.
But don't panic, it is reasonably simple. Now the action and the equal but opposite reaction are not measured in speed, they are measured in momentum. Because conventional propulsion won't work in space. So a rocket engine is just a way to fire some "reaction mass" (propellant) out the exhaust nozzle, so the recoil kicks the spaceship forwards. You will go sliding backwards on the ice or overboard the edge of the raft, with your dislocated shoulder.īut you will really move backwards fast if you are in a place with no friction. You can see this even better if you are standing on something with little friction, like on a piece of glare ice or in a raft on the water. 577 Nitro Express bullet out of your rifle, the rifle's recoil is going to kick like a mule and dislocate your shoulder. In English this means things will recoil. In a physics textbook Newton's third law says for every action there is an equal and opposite reaction.