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Homework Help: Rocket questions, Fundamentals of Physics

  1. Apr 5, 2005 #1

    I am confused with a solution of momentum in Rocket problems which published and solved in "Fundamentals of Physics, (Halliday-Resnick)".

    There are two different approachs to solving two similar questions. As i noticed, only difference is, the thing which flings. Fuel or tank(or any part of rocket). This is really confusing.

    Say, the rocket and fuel total mass is "M", fired fuel mass "m" and "v" is first velocity of rocket plus fuel, relative to earth. The fuel is firing backward with "u" relative to rocket body. For earth final velocity of rocket body is "v'". The equation is;

    M v = (M - m) v' + m (v - u) ....... (1)

    But if the thing which throws backward is the empty fuel tank beside of fired fuel then equation is,

    M v = (M - m) v' + m (v' - u) ....... (2)

    highly similar. But notice the last parentheses. This questions are at the end of chapter 9. And the solutions are in the solutions book.

    Chapter 9 - 6 : Preservation of Linear Momentum, exercises, question 15


    Chapter 9 - 7 : Practice of Momentum Principle, exercises, question 16.

    these two questions are friend but equations not similar.

    I am trying to guess about this difference but dont have any reasonable idea. Meanwhile, the questions,

    9 - 7 - 18 (exercises): equation looks like (2) above
    9 - 6 - 12 (problems) : equation looks like (2) above

    are related too. Tired of thinking. Can you see the reason? Or any idea.

    Note: This language is not my native for me. So, i tried my best for translating the name of chapters. BTW, i suppose the Chapter numbers are enough. If these explanations not enough then i will try to find them in English somewhere.

    Have nice day
  2. jcsd
  3. Apr 17, 2005 #2

    This question type is confusing me.

    Initially A and B is staying quiet together. Suddenly, after a blast, both far away from other. Say, B is going to positive direction, A is negative. Relative to B, start velocity of A is "Vba". Thus what is the velocity of B? (Ma, Mb : mass)

    Pi = Pf -> (Ma+Mb)0 = Ma Va + Mb Vb and Vba = Va - Vb then Va = Vb + Vba,

    0 = Ma (Vb + Vba) + Mb Vb then Ma (Vb + Vba) = - Mb Vb,

    After this equation, if we put the data in places, can solve the Vb.

    I know question is easy. And my solution is true. However hard to explain without pen-paper.

    Now say; A: Rocket Fuel, B: Rocket Body. Then above solution is wrong. It must,

    Vb = Ma/Mb x Vba

    for the last equation, look like "Vba" is not relative to rocket body but relative to quiet observer. But why? Why approachs are so different? I get this equation from Serway and "Fundamentals of Physics".

    Honesty, was not easy to explain for me with my poor English. But, this is my first try to ask a Physics question in internet. If my statements needs more explanations feel free to say.

    BTW, if you think is will be better to understanding then show me a link or maybe a name of book.

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