# Simple physics question

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#### Jimmyboy

##### Guest
For a car to move the wheels turn, plus the friction on the road and it goes. For a shuttle to enter orbit, rockets fire and the propelent pushes it off the ground and into the air. If an space craft is in deep space, it then fires its rockets how does it move?? there is no friction in space.. i know of newtons law every action has an equal and oposite reaction but i cant see how it works here. I understand how a solar sail would work because that is being given a push from the photons which are already traveling into it from the sun. Is it rockets do work but not very much because there isnt much matter in space voids to push off of, hence you dont see space crafts or probes using them to go to mars say, do they just glide there????

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#### SpeedFreek

##### Guest
Jimmyboy":3awnxk7x said:
For a shuttle to enter orbit, rockets fire and the propelent pushes it off the ground and into the air.

Not quite - the rocket is not pushing on the ground. The rocket "pushes" on the exhaust gases that are coming out of the back. For every pound of thrust pushed out of the rocket, an equal and opposite pound of thrust pushes the rocket forwards.

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#### Jimmyboy

##### Guest
And does that go for the same in space?

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#### MeteorWayne

##### Guest
Exactly. The ground (or air) provides no significant surface for the rocket exhaust to push against. It's all really how much mass and velocity you throw out of the back end, the body producing the thrust moves the opposite direction with the same energy.

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#### Jimmyboy

##### Guest
Yes but isnt space a vacum, there must be something the matter coming out of the back end is reacting with surely??

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#### SpeedFreek

##### Guest
Nope, the fact that matter is moving out of the back of the rocket is what moves the rocket forwards. Action, with an opposite reaction.

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#### MeteorWayne

##### Guest
No, not at all. Please spend a little time understanding the rocket equation before posting this in the Physics Forum. Otherwise it is headed for the Unexplained. This is basic 5th grade physics, not comlicated at all. Make the effort to at least understand the basics, OK?

J

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#### MeteorWayne

##### Guest
Read my response. This is simple 5th grade physics, so the question was quickly answered at that level.

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#### Jimmyboy

##### Guest
so people cant post subjects without you trying to be patronising, you have got problems meteor, especialy with someone who thinks there as clever as as you pal

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#### andrew_t1000

##### Guest
My 2 cents worth!
I had been told by a physicist that the thrust is actually produced in the combustion chamber, same with a jet engine.
Is this correct?
We were discussing this over a lot of beer!

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#### drwayne

##### Guest
Certainly the initial energy is imparted to the combustion products at that point. The nozzle in turn channels and controls the expansion of the gases subsequent to combustion. In fact, the nozzle shape is optimized depending
on where the rocket operates (low altitude, high altitude, space) to heep the exhaust plume from over or under
expanding.

Wayne

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#### drwayne

##### Guest
The simplest dscription of a rocket is this. Imagine someone in the back of a rocket, shooting a gun
out the back. Momentum is conserved, so the man (and the rocket he is attached to) recoils, which
in this case moves it forward.

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#### centsworth_II

##### Guest
Imagine an astronaut floating in space throwing a hammer. The force he uses in throwing the hammer creates an equal and opposite force on his body, pushing him in the opposite direction of the hammer.

Now imagine that the rocket engine is throwing gas (or plasma) out of the rocket. The force used to throw out this exhaust creates and equal an opposite force on the rocket, pushing it in the opposite direction of the exhaust.

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#### Jimmyboy

##### Guest
thanks dr/cents.... the shotgun analogy explains it well

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#### Mee_n_Mac

##### Guest
Just to add to drwaynes explanation ...

In case you want to think of it according to Newton's laws, you can try to resolve the forces involved and then from F=MA you can figure out what happens. Of course in a rocket the M is changing as you throw mass overboard but still the concept remains valid. Let's start with an inflated balloon. You blow it up and pinch off the end (lets call this end the back end). If the back remains pinched off the forces internal to the balloon are the same everywhere. The gas molecules inside push just as hard to the right as the they do the left, up the same as down, front = back, etc, etc. Everything is in balance and so the forces cancel out. The net F = 0 so there's no A and so no motion.

Now open the pinched end. Gas rushes out through the hole. The force on the balloon at the opening is lower that the force on the balloon directly opposite it. While all the other forces remain in balance (right = left), there's now a net difference between the forces to the "front" and to the "back". With some nonzero net force there's now an acceleration and thus motion.

In the case of a shotgun when the charge goes off gas is created in the barrel. It pushes "sideways" on the barrel itself (equally outward trying to expand the barrel) and on the shot column and on the breach face. The push on the breach face pushes on the stock and eventually onto your shoulder where you feel the recoil. Since you are "big" relative to this force, you accelerate a little to the rear. The forces on the shot column are bigger (relatively speaking) and thus it accelerates pretty hard (up to almost supersonic velocities) until the column exits the barrel and the gases pushing it can "escape" out to the sides.

A rocket is like the balloon but with a burning propellant that makes a lot of (hot) gases (like the shotgun). While it's easier to compute what happens using momentums, the basic F=MA (where M and A are both variables) equation still holds and makes commonsense.

Hmmmm, not so sure this actually makes things any clearer. Oh well ....

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#### drwayne

##### Guest
Note that it is not uncommon to have problems in Physics that can be looked at, and solved, in more
than one way. I tend to look at rockets as a conservation of momentum problem. Others might use an
analysis of the forces involved. They are seperate approaches that get to the same point. (The answer)

Wayne

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#### GreyMatters

##### Guest
MeteorWayne":3df5q80y said:
No, not at all. Please spend a little time understanding the rocket equation before posting this in the Physics Forum. Otherwise it is headed for the Unexplained. This is basic 5th grade physics, not comlicated at all. Make the effort to at least understand the basics, OK?

Wow. So thoughtful. So brilliant. I bet the this person will come back for more of your "infinite knowledge". Good Job!

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