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Homework Help: Bullet and pendulum bob initial speed help! momentum.

  1. Dec 11, 2007 #1
    bullet and pendulum bob....initial speed...help! momentum.

    1. The problem statement, all variables and given/known data

    An 7.0 g bullet is fired into a 3.5 kg pendulum bob initially at rest and becomes embedded in it. If the pendulum rises a vertical distance of 8.0 cm, calculate the initial speed of the bullet.
    ________m/s


    2. Relevant equations

    well there's many ways to do it, but heres one way:
    v=sqrt((1+M/m)*2gh)

    i tried it another way too, and got the same answer that i did with the above equation

    3. The attempt at a solution

    so first, 7g bullet = .007kg bullet and height of 8cm = .08m

    ok so then v=sqrt((1+3.5/.007)*2*9.8*.08) = ~28.03 m/s

    now, i tried this TWO WAYS and got the same answer.
    but webassign says it's wrong.

    why?
     
  2. jcsd
  3. Dec 11, 2007 #2

    Kurdt

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    The velocity of the pendulum and the bullet is given by:

    [tex] v=\sqrt{2gh} [/tex]

    The mass shouldn't enter into that part. Once v is known one can use the conservation of momentum to find the bullets initial speed.

    EDIT: I see what you've done to include the mass. The problem is the mass part should be outside the square root.
     
  4. Dec 12, 2007 #3
    which mass are you talking about?
     
  5. Dec 12, 2007 #4

    Kurdt

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    The (1+M/m) term.
     
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