# Space Acceleration?

Hello, I just began reading 2001: A Space Odyssey by Arthur C. Clarke and I have some questions. First of all I am a 16 year old junior so i haven’t taken any physics classes, so excuse me if my questions seem absurd =).

In space, what does a rocket push on? If there is on air behind it what exactly pushes a space shuttle and how does it accelerate.

My other question is: In space what stops a ship from going at whatever speed it can? If there is no drag, why can’t a space shuttle just turn on the rockets and accelerate to whatever speed it wants. In my non-physics oriented mind it seems like a space ship should be able keep on accelerating forever or until the speed of light. So what keeps a ship from just constant acceleration?

Thanks for reading and sorry for any type-o’s

A rocket pushes on the stuff it is flinging backwards - put on some roller skates and throw a bowling ball as hard as you can - you’ll find the ball goes one way and you go the other (regardless of whether the ball actually goes on to hit something)
Rockets work the same way - hurling out gases at high speed in one direction causes the rocket to be pushed in the other (like the recoil of a gun.

a) A rocket ship basically pushes on the gases that are expelled out of it’s exhaust, Newton’s principle of action-reaction means this imparts forward motion to the ship.

b) Relativity. Basically put a ship will gain (relativistic) mass (from the point of view of a ‘stationery observer’) the faster it goes, a spaceship travelling at the speed of light would have infite ‘mass’ and require infinite energy in order to accelrate it anymore, so the speed of light can never be obtained, only approached. There are other effects such as time dialtion to be considered but basically they are just restatments of the same thing.

Nothing. What we have here is one of Newton’s laws of motion, which can be simplified to “For every action, there is an equal and opposite reaction.” The rocket expels exhaust in one direction, which causes the rocket to accelerate in the opposite direction.

It’s conservation of momentum. If you were floating in space, holding, say, a bowling ball, and you decided to throw the bowling ball in one direction, you would move in the opposite direction. Now replace “bowling ball” with “rocket exhaust” and “you” with “space shuttle”.

The confusing branch of physics known as relativity. As an object accelerates, it gains mass. As an object gains mass, it requires more energy to accelerate it by the same amount. An object accelerated to the speed of light would have infinite mass, and thus require infinite energy to have accelerated it to that speed.

To have two typos in the word “typos” is one of the most ironic things I’ve seen in a long time.

When you fire a cannon there is recoil, right? Flinging the chunk of metal and hot gas out the end of the tube causes the cannon to try very hard to go in the opposite direction. The recoil of the cannon does not get less as you go up a mountain and the air get thinner. It would not get less in a vacuum, either. An interesting thing is, these “pushes” (the cannonball in one direction, the cannon in the other) are exactly equal and opposite. The forces, if combined, they would equal exactly zero, which makes sense when you think about it. All the mass started out at rest with respect to the other mass… the force that was applied to the cannonball by the expanding gas pushed hard on the ball, but equally hard on the cannon. The ball has less mass and so accelerated more rapidly, but the total force on each was equal.
This rule holds true whether there is air or not. The rocket acts by flinging mass opposite the direction it wants to go. The mass is the residue of the burned fuel, the rocket exhaust. Air to push against is not needed, and would only provide drag.
At the speeds we are talking about with rockets (not an appreciable fraction of the speed of light) you are correct that there would be no reason not to accelerate indefinitely, provided you did not run out of fuel. However, that’s a big if. Fuel weighs something. And, the tanks to hold it add more weight, and you need bigger pumps, etc. Recall that the cannon did not accelerate as rapidly as the ball, in fact moving very little, because it had much more weight. The more fuel you carry, the more you weigh and the less acceleration you get from a given thrust. Carrying enough fuel to accelerate for a long time may mean it just takes you much longer and more fuel to get up to the same speed. Note that spacecraft designers spend huge amounts of effort to shave a few ounces out of anyplace they can, packing \$1,000 titanium screwdrivers instead of a \$2 job that works just as well because it weighs less.

For your first question, a rocket in space pushes against the exhaust it’s throwing out the back. Picture yourself sitting in the middle of an ice-skating rink, holding some rocks. The ice has very little friction, so you can’t crawl to the edge. But if you throw a rock one way, you’ll go the other way. The same sort of thing happens with a rocket: It throws exhaust one way, it goes the other.

For your second question, the only thing stopping you from accelerating forever is that eventually, you run out of fuel. Practically speaking, this happens pretty quickly. You can’t just carry enough fuel to get your payload where you want it (and going the speed you want), because you also have to carry the rest of your fuel. So you have to carry more fuel to accelerate that fuel, and so on. Look at a diagram of a Saturn V some time: That little tiny cone on top is the payload, and pretty much everything else on the rocket is just fuel.

Incidentally, if you do have enough fuel, the speed of light doesn’t stop you from accelerating forever. You’ll always be going less than the speed of light, even if you’re always accelerating. Someone outside your ship will see your acceleration go almost to zero, but you’ll still feel yourself being pulled into the back of your seat in just the same way.

I am not a physicist either, but I’ll play the role of dabbler.

1. The rocket pushes on the hot fuel it’s dumping out the back end. Its not really ‘pushing’ on anything. Its relying on a neat property of momentum.

Demonstration: Pick up an object heavy enough that you can hold it at arms length but that is still fairly hefty. (Let’s say a heavy stone) Hold it against your chest. Push it away from you as hard as you can. You’ll go backward while it goes forward.

Without resorting, for the moment, to equations: The faster you push it away from you, the faster you will push yourself away from it. Alternately, the faster you push it away from you, the less massive it has to be to achieve a certain amount of velocity of you backward.

Fuel being dumped out of the back of a rocket is extremely hot and gaseous and made to point the opposite direction you don’t want to go as its expanding. It goes backward. It goes backward really really flipping fast. (And the faster you can make it go, the less of it you have to dump out the back). Consequence: You go forward.

This doesn’t require any surrounding ‘air’ or ‘ground’ to push against. You’re pushing against the fuel (also known as reaction mass).

1. In classic Newtonian mechanics, nothing. There’s a basic relationship to the amount of force you generate in a particular direction, your mass, and velocity. And this gives a pretty good measurement of the ‘reality’ of accelerated motion even up to appreciable fractions of the speed of light.

Barring hitting a planet, nothing will slow you down.

The speed of light isn’t a magic wall you hit. What happens is that as you approach the speed of light relative to some point you’re measuring that speed from (frame of reference is important in these discussions) you end up having time dilation effects and it appears to an observer back at your spacedock that you’re either getting more massive (so your thrust doesn’t accelerate you as much) or that you’re not putting out as much force (time dilation?).

These effects only start being measureable when you get to a serious fraction of the speed of light. You, aboard the spacecraft, can’t tell anything’s changed, and as far as you’re concerned, you’re continuing to accelerate at the same rate. So you’re not feeling any resistance from ‘the universe’ really.

Welcome to the counterintuitive nature of relativity (and there are dozens of physicists here willing to give you a much more detailed analysis of this problem).
From a practical standpoint, you can’t keep constant acceleration forever for a few reasons. One, you don’t have unlimited fuel to produce the force that causes the acceleration.

Two, the longer you want to burn the engines the more fuel you have to have, which increases your mass, which causes you to need more fuel to cause the same acceleration, which increases your mass, etc. If you work backwards in time from an empty tank given a (fairly simplistic) linear relationship between fuel and thrust, and dictate you want to maintain constant acceleration, you find there is a point at which your ship cannot carry enough fuel to push itself and its own fuel at that fixed acceleration.

This last point is somewhat esoteric but its a neat mathematical anomaly I ran across designing a fictional spacecraft, oddly. Its moot… you’d probably design for constant thrust, not constant acceleration. But you still don’t have infinite fuel for an infinite burn.

And again, you eventually run up against relativitistic effects, which complicates understanding what speed you really can get up to.

Gawd, I’m slow. I rant for a while, and everybody beats me by a mile.

I’ll just add a few things on which I was not clear on:

In special relativity all velocities are relative, there is no such thing as an absolute velocity; you cannot say “so and so is travelling with velocity, v”, you can only say “so and so is travelling with velocity, v, with respect to what’s his face”

relativistic mass is an outdated and rarely used concept, when a scientist says mass he means invariant mass which doesn’t change with velocity.

I’m probably misinterpreting you, but I’m going to assume I’m correct, just for the sake of argument. To me, it seems that your first point was covered well, but the answers to the second may have missed the point.

It’s possible you weren’t after relativity, or even fuel limitations. To me, you seem to be asking what happens to acceleration in the absence of air resistance. If there’s no drag from passing through the air, and no gravity pulling you to the ground, can’t you just go whatever speed you feel like going?

The answer is inertia. This is the tendency of all matter to resist changes in motion. This is often stated as Newton’s First Law of Motion (or the Law of Inertia) thus: “An object at rest will remain at rest, and an object in motion will remain in motion, unless acted on by an external force.” If an object were actually alone in outer space, without any other objects to act upon it (by gravitation or any other means), it would keep on going at the exact same speed, in the exact same direction, forever. In order to change either the speed or direction of its motion, a force must be applied.

With the space shuttle, the main force we’re concerned with is that offered by Newton’s Third Law of Motion (the action-reaction thing explained in other posts). Newton’s Second Law of Motion tells us that that force is proportional to the mass involved. So, accelerating tiny molecules (masses) of exhaust to very high speeds provides a certain amount of force, which, when applied to the space craft, makes it’s much larger mass accelerate a small amount.

OK, I can see I’m starting to go back over stuff from other poster’s stuff. I’ll stop. All I’m trying to get out here is that the mass of the space shuttle has an inherent property that keeps it from accelerating to any arbitrary velocity when acted on by a force. The force, mass and acceleration are related by the formula (you’ll get this on your second day of physics class) F = ma. Air resistance is a type of force, but the formula applies with or without air.

While it’s easily deducible from all the previous responses, I’d just like to point out that this isn’t some special aspect of rockets in space. The principle is the same for jet aircraft or rocket cars operating on and above earth. They just have to fight wind resistance, too. (A jet airplane won’t work without air, however, because it makes use of the oxygen in the air for fuel.)

A propeller plane does in some sense ‘push’ on the air (at least the prop does), though it makes as much sense to think of it in terms of throwing the air behind it, similar to a jet or rocket.

Of course, with a prop or other engine pushing air like that, you do have other forces involved, don’t you? The buildup of increased pressure behind the prop, and the vaccum in front. I would presume this pressure difference affects thrust where in space you rely on momentum alone.

Of course, with a prop or other engine pushing air like that, you do have other forces involved, don’t you? The buildup of increased pressure behind the prop, and the vaccum in front. I would presume this pressure difference affects thrust where in space you rely on momentum alone.