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pratu043

  • 2 years ago

I have a doubt in the answer key to the problem on law of conservation of momentum. The question: A girl of mass 50 kg jumps out of a rowing boat of mass 300 kg on to the bank with a horizontal velocity of 3 m/s. With what velocity does the boat move backwards? Answer key says: m1(mass of girl) = 50 kg m2(mass of boat) = 300kg v1(velocity of girl) = 3 m/s v2 = ? m1v1 + m2v2 = 0 How can the momentum of the whole system after the girl jumps out of the boat be 0? The boat will have some momentum right?

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  1. Xishem
    • 2 years ago
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    Right. The boat must have some change in velocity, since the momentum must be conserved and the momentum of the girl changed.

  2. Yahoo!
    • 2 years ago
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    it is just same as recoil velocity of Gun..

  3. pratu043
    • 2 years ago
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    but even in recoiling of gun, how can the final momentum be 0?

  4. Yahoo!
    • 2 years ago
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    v = - Mv/m = -50*3/300 = -0.5m/s

  5. pratu043
    • 2 years ago
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    Yes, I know the answer but I can't understand how total momentum is 0.

  6. Yahoo!
    • 2 years ago
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    Law of conservation of momentum..says that momentum is conserved

  7. Yahoo!
    • 2 years ago
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    Do..u knw abt Rocket propulsion

  8. Xishem
    • 2 years ago
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    Momenta are vectors.

  9. Xishem
    • 2 years ago
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    If you have two vectors in opposite directions, they can cancel eachother out.

  10. Yahoo!
    • 2 years ago
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    even though the mass gets reduced..the momentum is conserved

  11. pratu043
    • 2 years ago
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    even if they are different?

  12. Xishem
    • 2 years ago
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    The girl's momentum is in one direction and the boat's in another direction.

  13. Yahoo!
    • 2 years ago
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    yup..@Xishem has a point there

  14. pratu043
    • 2 years ago
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    I mean do they cancel each other even if the magnitudes are different?

  15. Xishem
    • 2 years ago
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    The magnitudes aren't different.

  16. Xishem
    • 2 years ago
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    Remember, momentum is a function of both mass and velocity.

  17. Xishem
    • 2 years ago
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    If one object with 1kg mass and 10m/s velocity is moving right and another of 10kg mass and 1m/s velocity is moving left, the total momentum is zero.

  18. pratu043
    • 2 years ago
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    So mass*velocity of the bullet or girl will always be equal to mass*velocity of gun or boat?

  19. Xishem
    • 2 years ago
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    Yes, assuming you are in a closed system. Which you are.

  20. pratu043
    • 2 years ago
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    What is a closed system?

  21. pratu043
    • 2 years ago
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    Please help I have a test coming up soon.

  22. Xishem
    • 2 years ago
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    A closed system is a physical system which doesn't exchange any matter in or out of itself and which isn't affected by outside forces.

  23. Xishem
    • 2 years ago
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    It basically just means that you are only considering the two objects and not nonconservative forces like friction which would change the overall momentum. (This case is frictionless, but that's one case where momentum won't be conserved).

  24. pratu043
    • 2 years ago
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    Thanks I got it now.

  25. pratu043
    • 2 years ago
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    This doesn't happen in the case of a collision right?

  26. Xishem
    • 2 years ago
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    Momentum is not conserved in an inelastic collision. Momentum is conserved in an elastic collision.

  27. Xishem
    • 2 years ago
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    In the real world, momentum is never conserved in collisions. When two pool balls hit eachother for instance, they release some sound upon impact -- this is lost energy. They are also always under the effect of friction from the table.

  28. pratu043
    • 2 years ago
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    Thanks.

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