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Why is fuel still needed in space?

  1. Jul 25, 2006 #1
    Why is fuel still needed in space???

    If space consists on nothingness, why would a rocket need fuel to gain momentum? For example, if you were to hit a golf ball in space, it would keep going and going right? So besides a gravitational pull from the earth that the rocket initially needs to escape from, what other reasons are there for the fuel used for acceleration? My main question is, if the fuel is needed, how does it create a push for the rocket to gain velocity or turn when there is nothing to push from (i.e. air)???
  2. jcsd
  3. Jul 25, 2006 #2
    One theory was initially postulated by Mach it states:

    "The inertia of any system is the result of the interaction of that system and the rest of the universe. In other words, every particle in the universe ultimately has an effect on every other particle."
    (from wikipedia)

    However Mach's principle is not compatible with specal and general relativity.

    Well apart from Mach's theory there doesn't seem to much theory about it. :smile:
    Last edited: Jul 25, 2006
  4. Jul 25, 2006 #3

    I think, on the contrary, that Mach's principle is at the root of general relativity.
    General relativity precisely incorporates the hypothesis that (local) inertial frames are determined by the surrounding masses and not "ex-nihilo".

  5. Jul 25, 2006 #4


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    Conservation of momentum. Say the initial momentum is 0. By throwing something out the back (i.e. hot gas from your rocket), you can make the rest of the rocket go forward since the total momentum of the system must remain 0.

    EDIT: What's this doing in quantum physics?
    Last edited: Jul 25, 2006
  6. Jul 25, 2006 #5
    That is correct!
    But that is in my opinion (and I know that others may dffer on that) not relevant to Mach's principle. :smile:
  7. Jul 25, 2006 #6
    What on earth does Mach's principle have to do with rockets???

    childsy - Without any acceleration, an object in free space will simply keep moving in a constant direction at constant velocity - like the golf ball you mentioned. Momentum is conserved like that. But humans generally don't want to fly off into the distance, so they use rocket thrust to accelerate and change their momentum. The principle is the same as on earth; an engine accelerates hot gases out one end at high velocity, so the rest of the rocket moves the other end to compensate (to conserve momentum). The rocket loses some of its mass in the exhaust, so the COM itself is not moving.


    By the way, "Quantum Physics" is not a forum relevant to this question. Next time try "Classical Physics" or "General Physics".
    Last edited: Jul 25, 2006
  8. Jul 25, 2006 #7
    The frame dragging effect, due to an object's rotation in space, is actually being tested by Gravity Probe B.
    The result of that test will perhaps tell us something about the validity of Mach's principle.

    Gravity Probe B - Stanford website
    Last edited: Jul 25, 2006
  9. Jul 25, 2006 #8

    You are partly right. The Mach's principle is more an explanation for the Principle of Inertia.

    But it seems that you know very well what the Principle of Inertia is. Therefore, you understand that using a rocket will accelerate the space ship and allow it to reach faster its target.

    And where does the thrust of the rocket comes from?
    The Principle of Inertia, again, tells us that in any circumstance without external forces, a system will keep going at a constant velocity. Now, if this system disintegrates -so to speak- then the principle still applies, but to the center of mass of the system. Apply that to the "rocket-spaceship-hot gas" system and you understand that if the hot gas is accelerated to the back, then necessarily the ship must accelerate forth. This is called conservation of momentum: it is (partly) a consequence of the Principle of Inertia.

    Last edited: Jul 25, 2006
  10. Jul 26, 2006 #9
    god damit, the mas asked a question about rockets, and your all babbling about general relativity, and quaton physics... when the answer lies within old simple newton.
    why is it that people talk about nowdays physics, while not knowing basic mechanics.
  11. Jul 26, 2006 #10

    I agree with you that often on this forum, and elsewhere, people tend to discuss very advanced topics in physics while they are unable to explain how moppeds work.

    However, it was more difficult, for me at the secondary school, to understand the Principle of Inertia and what an Inertial Frame is, than to learn Maxwell's equations and quantum mechanics at university.

    I think that a short explanation of the Principle of Inertia can be based on the Mach principle and can be very helpfull to assimilate basic mechanics. One always assimilate better what we understand, specially for people who try to understand first.

  12. Jul 26, 2006 #11

    Space doesn't consist of nothingness. Only nothing consists of nothingness, and nothing is nowhere and never to be found :) One can't find nothing when it doesn't exist in the first place.
    Last edited: Jul 26, 2006
  13. Jul 26, 2006 #12


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    This thread started out on the wrong foot with MeJenniffer's response. It should have been done via straight-forward newtonian mechanics. Just apply the conservation of momentum, as eep has indicated.

    Thus, to achieve a gain in momentum, there must be "fuel" involved because the expulsion of gasses from the exhaust will cause a net momentum gain by the rocket to preserve overall linear momentum. This is similar to a person on ice throwing a ball - that person will gain a momentum in the opposite direction.

    Please try to stay ON TOPIC and not hijack or derail the thread by invoking irrelevant principles. Such things have seldom helped in answering the original question. Re-read the PF guidelines if you think you have forgotten them, because they ARE enforced.

  14. Jul 26, 2006 #13

    why is fuel used in space?
    first of all, the gravity on shuttles in space is just a little lesser than the gravity here on earth(unlike many people uneducated in science thinks... people are floating in shuttles only because there is no pull force between the person and the shuttle, but the man is still pulled to earth)

    so after the shuttle crossed the atmosphere, it needs to "resist" gravity, so they dont fall right back to earth, which is obviously a bad thing to happen.
    so the next step to be in space and not die right after, is to get the shuttle in to orbit, so that the shuttle may accelerate toward earth, yet not get closer, which is orbit motion, if u did not learn high school physics, it may be hard for u to understand what orbit motion actully is...

    now to start orbiting earth you will need to gain a certain tangental velocity(which is determined by gravity, and distance from earth). and this velocity is gained by the fuel burned.

    how does a shuttle moves itself?
    im no expert, and im not aware on the methods used to gain motion in space, but i know that rocket motors can do the trick, yet i am no expert about rocket =), but ill give you an example-
    the water pipe in ur garden may move itself only by splashing out water, just the fact that water gain motion out of the pipe, it means that they pushed something to gain that motion.
    same with the rocket motor, just with veeery hot gas.

    btw, i heard of a new method being developed this days, for sattlleites. the sattellite when in space, has this big screen, which reacts to photons and ions which are sent by the sun. that reaction is a force which moves the sattellite.
    its funny, all the motors we make with all the complex fuels we use, and in the we decide that a simple sail is the right sulotion.
    "aye aye cap'n!"
    Last edited: Jul 26, 2006
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