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Thread: F=MA

  1. #1
    MR Meek and Mild Epidemic's Avatar
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    F=MA

    For propulsion in a rocket engine what is force F.

    Is force generated by the atoms of fuel burned being thrust back so fast as to create forward motion?

    Is force a calculation of the mass of atoms thrust back in conventional rocket engine?

    Fellar here is telling me that you need force to move your spaceship and I just do not see force doing anything unless it acts on something.

  2. #2
    I am a banana! originel's Avatar
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    Force is created by the atoms of burning fuel thrust backwards, but mass in the F=MA equation is not the mass of the fuel atoms. Mass would be the mass of the entire rocket.

    Force can be expressed in many many forms. You could also calculate it using some chemistry equations for the ignition of the fuel (and using the chemical energy of it). MA is just the most easily available because it is independant of the type of force being used.

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    MR Meek and Mild Epidemic's Avatar
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    That is my take on it. But this guy in the office is arguing force is simply the conversion of mass into energy and he contends that the exaust gass is merely the byproduct of that.

    fuel energy I look at as F= the velocity one accelerates the propellant a given mass atom over the duration of acceleration.

    If accelerating a xenon atom to 1,000,000 MPH in 6 inches generates 1 gram of thrust then accelerating 1000 xenon atoms to 1,000,000 in six inches of the nozzle will generate a Kilogram of thrust or force. Would that be correct.
    Last edited by Epidemic; June 21st, 2004 at 12:38 PM.

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    I am a banana! originel's Avatar
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    He's about half right. Force is not created by a mass that's accelerating. In our rocket example the actual force IS the burning of fuel which then exhausts. But the gas is not a byproduct, the exhaust gas is what actually creates the Force. We use the mass and acceleration mostly as an easy convention to calculate the force, It's all purely theoretical really and doesn't tell us anything about the nature of the force. To do it properly we would use the chemistry equations (which i do not know).

    thrust is not measured in grams, mass is. Force is measured in kg*m/s^2 (units of mass times units of acceleration) or N (newtons). also your calculations are not correct (units aside). I'm sorry but i don't feel like using your values because to derive the acceleration would recquire a system of equations, not to mention i'd have to dig out my chem book to find the atomic mass of xenon. It's summer and i shouldn't have to do linear algebra, chemistry, and physics in the same morning . but in any case i HIGHLY doubt you would get .001 as an answer (kG is used as the base unit of measurement for mass, not the gram).

    Let's use a 3kG block of something and forgetting about air resistance, friction, etc (which is also just a derivation of F=MA). now we want to move it 0.1524m again (6 inches). If we move it that far in 1/100 of a second using the equation x=x0+v0*t+1/2*a*t^2 (where x is distance, x0 is initial distance which equals 0, v0 is the initial velocity which also equals 0, t is time, and a is acceleration) that would give us an acceleration of 3048m/s^2. So the force would be 9144 N. Now let's consider the block moving the same distance but in 100 seconds this time. That would recquire a force of 9.144e-5 N.

    btw physics should ALWAYS be done in metrics, not SI. that's why that mars rover crashed into mars afterall .

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    MR Meek and Mild Epidemic's Avatar
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    I use SI measurements not for an accurate measure but rather simply to state that the acceleration of the fuel and the directivity of that fuel acceleration force aft direction is what propels you forward.

    Now he is killing me.

    If I have a closed loop with 1 liter of water and I through electrolysis convert it to hydrogen and oxygen after I heat the volatile mixture of Hydrogen and oxygen will I have the same volume of H20 I started with. If I do have the same amount of water in the
    end where did the energy come from. He contends that the H20 will be less volume because in the release of heat atoms are consumed.

    This guy is an degreed engineer with a firm chemistry background but I feel he is completely wrong on this as well. My assumption is that the electrolysis is freeing the H and O by adding electrons, when you burn it electrons are lost in the exothermic reaction

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    Senior Member Hungrycookpot's Avatar
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    atoms dont get "consumed", they always must be somewhere in some form. If you have a CLOSED loop, then there will ALWAY be the same amount of matter inside this loop. (im assuming you are talking about theorectically, because in practise its always possible to have some hydrogen leak out or something)

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    MR Meek and Mild Epidemic's Avatar
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    He contends they are consumed and released as thermal energy. In the theoretical closed loop there would be markedly less volume according to him as the heat was released.

    10 H2's
    5 O2's
    will make 10 H20's

    But in fairness if energy can not be created or destroyed where
    did the energy come from if nothing was lost in the process?

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    Senior Member Hungrycookpot's Avatar
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    from the electrolysis

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    Senior Member Hungrycookpot's Avatar
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    perhaps less volume, but thats only because a liquid is much more compact then a gas

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    MR Meek and Mild Epidemic's Avatar
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    No NO NO. once I split the water into hydrogen and oxygen I introduce heat to begin the reaction causing the O2 to combine with the hydrogen and create water.

    In the end after cooling will I still have a litre of water or will some of the atoms have been transformed to energy.

    Personally I know that the H20 volume will be the same in the end in this closed loop at least theoretically assuming complete recombination of 02 and H.

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    Senior Member Hungrycookpot's Avatar
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    yes i would have to agree with you, if it is closed and all the molecules reform as completely as possible, then you would have the same amount of water as you started with. The only way you can destroy molecules is in nuclear reactions, and even then if im not mistaken the mass is still present, just not in the form it was before

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    OAP Theophylact's Avatar
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    Yes, the H2O will be lighter than its component atoms by exactly the mass equivalent of the energy released in its formation from its elements. E=mc˛. But work it out, and you'll find the mass difference is many orders of magnitude too small to measure...

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    I am a banana! originel's Avatar
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    I use SI measurements not for an accurate measure but rather simply to state that the acceleration of the fuel and the directivity of that fuel acceleration force aft direction is what propels you forward.
    I see now...but you should still always use metrics SI is only good for creating error, once again referring to the mars rover. If they had used all metrics it wouldn't have crashed. There is an equivalent metric unit for every single SI unit and there are several metric units that have no SI equivalent. Besides, F=ma doesn't work for SI units, only metric units.

    If I have a closed loop with 1 liter of water and I through electrolysis convert it to hydrogen and oxygen after I heat the volatile mixture of Hydrogen and oxygen will I have the same volume of H20 I started with. If I do have the same amount of water in the
    end where did the energy come from. He contends that the H20 will be less volume because in the release of heat atoms are consumed.
    wow he's a complete idiot. There's no such thing as a "heat" atom. Heat is just energy that is transfered from atom to atom. The volume will actually go up since gasses are less dense than liquid at close to room temperature and pressure. The energy came from an outside source, such as an electric plug in the wall or whatever is powering the electrolysis. But yes in a perfect system you will have the same amount of water in the end as you had in the beginning

    He contends they are consumed and released as thermal energy
    energy is not a substance and atoms are not transformed into energy. atoms contain energy.

    Personally I know that the H20 volume will be the same in the end in this closed loop at least theoretically assuming complete recombination of 02 and H.
    exactly.
    Last edited by originel; June 21st, 2004 at 02:49 PM.

  14. #14
    MR Meek and Mild Epidemic's Avatar
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    Quote Originally Posted by Theophylact
    Yes, the H2O will be lighter than its component atoms by exactly the mass equivalent of the energy released in its formation from its elements. E=mc˛. But work it out, and you'll find the mass difference is many orders of magnitude too small to measure...
    OK theo,

    So are we talking about the electrons released during the formation of the water.

    and if so then these are the same electrons replaced during the electrolosys process?

    running the cycle over and over in the closed loop will always yield the same number of atoms right?

  15. #15
    MR Meek and Mild Epidemic's Avatar
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    Point of clarification
    in the end where did the energy come from. He contends that the H20 will be less volume because in the release of heat atoms are consumed.
    Change that to "in the end where did the energy come from. He contends that the H20 will be less volume because in the release of heat, atoms are consumed.


    he never claimed that there were heat atoms. But he does imply that matter is converted to energy (i.e. small scale nuclear reaction)

  16. #16
    OAP Theophylact's Avatar
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    Quote Originally Posted by Epidemic
    OK theo,

    So are we talking about the electrons released during the formation of the water.

    and if so then these are the same electrons replaced during the electrolosys process?
    No, we are not talking about the electrons released in the electrolysis process; there are exactly as many electrons left at the end as there were in the beginning, they're just localized on different atoms. You can't manufacture charge, it's totally conserved.

    We're talking about the energy released by combustion of hydrogen and oxygen to produce water; or conversely, the energy consumed in the electrolysis of water to produce hydrogen and oxygen. (It's exactly the same amount; only the sign is different.) You can calculate that from tables of heats of formation, or from the half-cell potentials.
    running the cycle over and over in the closed loop will always yield the same number of atoms right?
    Right but irrelevant. Although the number (and type) of atoms is conserved, the mass of the starting molecules and the product ones differ by a teensy amount, calculable by relativity to be the equivalent of the energy difference.

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    ph34r t3h g04t Whir's Avatar
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    What if you wanted to make heavy water?

  18. #18
    Fact Checker Gomer's Avatar
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    Rockets: In laymens terms...
    http://science.howstuffworks.com/rocket1.htm

    Read that and get back to me.

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    MR Meek and Mild Epidemic's Avatar
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    Thanks gomer

    As expected it is the mass of what is ejected which causes the reaction.

    Good site though. I went off in many directions from that original link.

    Fusion drive
    Ion drive
    solar sails
    Light drive

  20. #20
    OAP Theophylact's Avatar
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    Quote Originally Posted by Whir
    What if you wanted to make heavy water?
    Don't drink light beer...

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