The Lemon Fool

An incredibly naive question, but I wonder how in deep space (no gravity and no atmosphere) a rocket accelerates in what seems to be a vacuum. I.e. there seems to be nothing which the rockets can push against - if you see what I mean...

I’ll probably regret asking something so basic.

Jon

Rockets and engines in space behave according to Isaac Newton's third law of motion: Every action produces an equal and opposite reaction.

https://www.livescience.com/34475-how-d ... t-air.html

RC

ReformedCharacter wrote:Rockets and engines in space behave according to Isaac Newton's third law of motion: Every action produces an equal and opposite reaction.

https://www.livescience.com/34475-how-d ... t-air.html

RC

Very many thanks RC. Most interesting. I’m tempted to wonder whether Newton got it quite right* because in the way that I put my question there’s no actual ‘opposite reaction’. But I guess that I’ll just have to accept that it’s like the theory of relativity and so much else - something that I’ll never understand.

All the best

Jon
*denotes tongue in cheek.

i would imagine that in effect the thrust is pushing against itself ie its own hull rather than "air" outside of the craft?

didds

Jonetc15 wrote:

ReformedCharacter wrote:Rockets and engines in space behave according to Isaac Newton's third law of motion: Every action produces an equal and opposite reaction.

https://www.livescience.com/34475-how-d ... t-air.html

RC

Very many thanks RC. Most interesting. I’m tempted to wonder whether Newton got it quite right* because in the way that I put my question there’s no actual ‘opposite reaction’. But I guess that I’ll just have to accept that it’s like the theory of relativity and so much else - something that I’ll never understand.

All the best

Jon
*denotes tongue in cheek.

Imagine you were in space, and you fired a gun in one direction. The bullet would go one way, and you'd go backwards a little bit.

Now imagine the rocket is the bullet, and you're the fuel escaping from the back!

Jonetc15 wrote:An incredibly naive question, but I wonder how in deep space (no gravity and no atmosphere) a rocket accelerates in what seems to be a vacuum. I.e. there seems to be nothing which the rockets can push against - if you see what I mean...

I’ll probably regret asking something so basic.

Jon

F=ma

The thing you are thinking of as a 'rocket' is throwing a small amount of itself out the back end at very high speed, in order to accelerate the front end. It pays to pay attention to the back end's exhaust.

regards, dspp

Jonetc15 wrote:

ReformedCharacter wrote:Rockets and engines in space behave according to Isaac Newton's third law of motion: Every action produces an equal and opposite reaction.

https://www.livescience.com/34475-how-d ... t-air.html

RC

Very many thanks RC. Most interesting. I’m tempted to wonder whether Newton got it quite right* because in the way that I put my question there’s no actual ‘opposite reaction’. But I guess that I’ll just have to accept that it’s like the theory of relativity and so much else - something that I’ll never understand.

All the best

Jon
*denotes tongue in cheek.

Try sharing a swing with someone next time you’re playing in the local park. If they jump off in a certain direction then you go in the opposite direction! Similar things can happen if you’re in a small boat close to the shore and you make a jump for it (the boat moves away from the shore)

dspp wrote:

Jonetc15 wrote:An incredibly naive question, but I wonder how in deep space (no gravity and no atmosphere) a rocket accelerates in what seems to be a vacuum. I.e. there seems to be nothing which the rockets can push against - if you see what I mean...

I’ll probably regret asking something so basic.

Jon

F=ma

The thing you are thinking of as a 'rocket' is throwing a small amount of itself out the back end at very high speed, in order to accelerate the front end. It pays to pay attention to the back end's exhaust.

regards, dspp

To illustrate dspp's point simply: blow up a balloon and let it go of it ...

The air goes one way, the balloon t'other. Conservation of momentum in action.

If you envisage the rocket and the rocket fuel as a whole it has a centre of mass (centre of gravity), as the fuel is fired out of the back at very high speed the rocket moves forward, however the centre of mass remains in the same place.
Basically a large amount of fuel going at a very high speed in one direction and a rocket going much slower in the other direction.
As someone noted earlier Mass times acceleration in one direction equals Mass times acceleration in the opposite direction.
As an indication the total weight of a saturn V was 2.8 million kilograms, this could put 100,000 kg in low earth orbit and deliver 50,000 kg to the moon (this includes lunar orbiter and lander and fuel for the return journey) and returned 14,690 kg to earth.

Just a couple of points. Mostly been answered already.

Jonetc15 wrote:An incredibly naive question, but I wonder how in deep space (no gravity and no atmosphere) a rocket accelerates in what seems to be a vacuum. I.e. there seems to be nothing which the rockets can push against - if you see what I mean...

I'm not sure what you consider "deep space", even the Moon isn't really what I would think of as "deep space"! BTW there is as much "gravity" in "deep space" as anywhere else, it isn't a peculiar property of Earth's surface and doesn't just disappear because you leave Earth - it's gravity that keeps the Moon in orbit around the Earth and the Earth in orbit around the Sun etc.

Rocket propulsion works better in 'empty' space, no air to get in the way!

A pure jet engine works on exactly the same principle as a rocket engine just that, unlike the rocket, it only needs to carry its own fuel around, the oxidant comes free, as air. But many/most(?) jet engines - certainly commercial airline turbo-fans - are not 'pure' and use a jet engine which is 'geared up' by driving a large air fan. So kind of like a propeller aircraft in disguise. But again, it all comes down to much the same physical principles.

One thing the OP might not have considered, is that rockets only need fuel to accelerate.

Unlike on Earth, there is nothing in space to slow the rocket down, so a small amount of fuel burnt and sent out the back at high speed will cause the rocket to start to move in the other direction, and it will simply keep on going at the same speed until it hits something (at which point that object will take some of the kinetic energy from the rocket). Burning more fuel simply makes it move faster so quite small amounts of fuel are required to travel huge distances.

Paul

Jonetc15 wrote:An incredibly naive question, but I wonder how in deep space (no gravity and no atmosphere) a rocket accelerates in what seems to be a vacuum. I.e. there seems to be nothing which the rockets can push against - if you see what I mean...

I’ll probably regret asking something so basic.

Jon

When I was at school 50+ years ago and the whole Apollo program was big news, this very same question came up and my physics teacher came up with a simple experiment that we could do to help our understanding of Newton's 3rd law of motion.

She borrowed a medicine ball from the gym and we went over to the park across the road.
One person sat on a swing with the medicine ball and waited until it stabilised.
They then pushed the ball away from them, which caused them to move the other way.

A rocket, firework night or Apollo sized works by throwing away lots and lots of very small particles in the same direction, very, very, quickly, causing the rocket to move in the opposite direction. Gravity and atmosphere are just getting in the way.

Slarti

Slarti wrote:When I was at school 50+ years ago and the whole Apollo program was big news, this very same question came up and my physics teacher came up with a simple experiment that we could do to help our understanding of Newton's 3rd law of motion.

She borrowed a medicine ball from the gym and we went over to the park across the road.
One person sat on a swing with the medicine ball and waited until it stabilised.
They then pushed the ball away from them, which caused them to move the other way.

A rocket, firework night or Apollo sized works by throwing away lots and lots of very small particles in the same direction, very, very, quickly, causing the rocket to move in the opposite direction. Gravity and atmosphere are just getting in the way.

Very good.

Did she also attempt an answer to that other question that "came up" at the time? 'Why is there no gravity in space?'

XFool wrote:

Slarti wrote:When I was at school 50+ years ago and the whole Apollo program was big news, this very same question came up and my physics teacher came up with a simple experiment that we could do to help our understanding of Newton's 3rd law of motion.

She borrowed a medicine ball from the gym and we went over to the park across the road.
One person sat on a swing with the medicine ball and waited until it stabilised.
They then pushed the ball away from them, which caused them to move the other way.

A rocket, firework night or Apollo sized works by throwing away lots and lots of very small particles in the same direction, very, very, quickly, causing the rocket to move in the opposite direction. Gravity and atmosphere are just getting in the way.

Very good.

Did she also attempt an answer to that other question that "came up" at the time? 'Why is there no gravity in space?'

The simple answer, that there is, but in orbit everything is falling at the same speed so it feels as if there is no (or very, very, very little) gravity

Slarti

Strictly accelerating at the same rate remembering that velocity has direction as well as magnitude. Get too close to a black hole and then the variation of gravitational forces would be noticeable.

johnhemming wrote:Strictly accelerating at the same rate remembering that velocity has direction as well as magnitude. Get too close to a black hole and then the variation of gravitational forces would be noticeable.

If you were responding to my
"The simple answer, that there is, but in orbit everything is falling at the same speed so it feels as if there is no (or very, very, very little) gravity"

The word orbit was the clue, in a stable orbit you are falling at just the right speed that you go round. You are not accelerating at all.

Slarti

Acceleration is a change in velocity not speed. Orbiting objects do not feel the effect of gravity because everything in the object is accelerating at the same rate. In fact if an object is in an eliptical orbit its speed (being a directionless aspect of velocity) varies as it moves towards and away from the object that it is orbiting. It is difficult to have orbits that are perfectly circular.

Slarti wrote:The simple answer, that there is, but in orbit everything is falling at the same speed so it feels as if there is no (or very, very, very little) gravity

That's not the (ahem! cough!) 'answer' I heard in the 1970s during a TV Apollo programme that had a question & answer session for children. One of the experts(?) was asked the question and I always remember the answer he gave:

"As you get further from the Earth its gravity gets weaker." OK

"As you get nearer the Moon its gravity gets stronger." OK

"At some point the Earth and the Moon's gravity cancel each other out." OK

"That's the reason." ???

"Err... Excuse me, sir!"

Poor chap; possibly, when faced with this question from children and little time to answer and no diagrams, he just opted for an easy way out? I've often wondered since.

Slarti wrote:

johnhemming wrote:Strictly accelerating at the same rate remembering that velocity has direction as well as magnitude. Get too close to a black hole and then the variation of gravitational forces would be noticeable.

If you were responding to my
"The simple answer, that there is, but in orbit everything is falling at the same speed so it feels as if there is no (or very, very, very little) gravity"

The word orbit was the clue, in a stable orbit you are falling at just the right speed that you go round. You are not accelerating at all.

You MUST be "accelerating" when in an orbit, as you are changing velocity (speed/direction).