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Homework Help: Balistic Pendulum

  1. Dec 6, 2008 #1
    1. The problem statement, all variables and given/known data
    An 8.0g bullet is fired into and becomes embedded in a 2.5kg block of wood at the end of a pendulum. The block of wood swing to a height of 6.0cm. (Watch Units)
    a) What is the PE of the block & bullet at the top?
    b) What is the KE at the bottom? (COE)
    c) What is the velocity of the block & bullet right after the impact?
    d) What is the pmomentum at the bottom?
    e) What was the initial speed of the bullet? (COM)

    2. Relevant equations
    KE= PE
    KE= 1/2 mv2
    PE= mgh

    3. The attempt at a solution
    a)PE = (.008kg + 2.5kg)(9.8 m/s2)(.06m) = 1.47 J (doesnt seem like a reasonable number to me)

    b)KE = .5(.008kg + 2.5kg)(v)2

    c) mgh=1/2mv2

    v=(square root)2gh
    (square root) (2)(9.8m/s2)(.06m)=1.08 m/s

    d)Does "bottom" mean not moving? if not, then
    P=mv P=(.008kg+2.5kg)(1.08m/s)=2.71 kgm/s

    e)(.008kg)(v1) + (2.5kg)(v2) = (.008kg + 2.5kg)v'
    .008kg(v1) + 2.5kg(v2) = 2.508kg(v')
    Last edited: Dec 6, 2008
  2. jcsd
  3. Dec 6, 2008 #2
    figured it out
  4. Dec 6, 2008 #3


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    Homework Helper

    EDIT: I was in the process of typing this up while you figured it out. You may still find my first comment useful, though.

    This is too vague (i.e. generally untrue). I would advise against ever using this equation because it is imprecise and can cause confusion. I suggest the more explicit:


    from which you can derive


    by recognizing that KEf=0 (at the top of the swinging motion of the block) and PEi=0 (by simply choosing that as your zero point for PE, since gravitational PE is relative to your choice of zero).
    Notice that the KE in this equation corresponds to a different situation than the PE, which I have indicated with the identifiers, "i" for initial and "f" for final.

    Alternatively, try:

    (is KEf=0)

    Why does this seem unreasonable to you? I got the same thing, except for a sig fig issue. If you need to be picky about sig figs, then how many should you have?

    No. Use the hint, "(COE)". This is basically how you solved part (c), so you obviously know how to do this. Think of part (b) as an intermediate step between parts (a) and (c); I think that was the point of part (b).

    Probably so, since it specifies "after the impact".

    Any guesses what is v2 and v'?
  5. Dec 6, 2008 #4
    would PE be KE also, as part b as 1.47 J?

    and for part e I ended up getting 339 m/s, which seems reasonable for a bullet
  6. Dec 6, 2008 #5


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    Yes. And it looks like you're doing fine with these problems, but I still strongly suggest that you take heed of my first suggestion in my previous post. (That includes the use of more specific identification, such as "i" and "f".)

    It seems reasonable to me, too. :)
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