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Pendulum Problem: AP Physics 1985B1

  1. Jan 2, 2009 #1
    A 2-kilogram block initially hangs at rest at the end of two 1-meter strings of negligible mass as shown on the diagram above (I'll explain this later). A 0.003-kilogram bullet, moving horizontally with a speed of 1000 meters per second, strikes the block and becomese embedded in it. After the collision, the bullet/ block combination swings upward, but does not rotate.
    The diagram shows pretty much everything described above. It doesn't give how far apart the strings are from each other/ the block, etc.
    a. Calculate the speed v of the bullet/ block combination just after the collision.
    b. Calculate the ration of the initial kinetic energy of the bullet to the kinetic energy of the bullet/ block combination immediately after the collision.
    c. Calculate the maximum vertical height above the initial rest position reached by the bullet/ block combination.

    m1v1 +m2v2 = m1v1' + m2v2'
    KE= 0.5 m v^2

    I got all the answers using the kinetic/potential energy equations above, and assuming that only one string was holding up the block (like a pendulum). I could write out the work I did to get these answers if you would like. But my main question is: how does having two strings attached to the block affect my answers? I think that, if you attach more strings, and attach the strings closer to the ends of the block, the block will move less. But the problem doesn't give any string measurements in relation to the block - only that the string itself is 1m long.

    I would appreciate any ideas or insight. Thanks in advance!

    p.s. How do you use the template when starting a topic? thanks!
  2. jcsd
  3. Jan 2, 2009 #2


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    Homework Helper

    I can't see your diagram, but I suspect the idea of the 2 strings is that it will prevent lateral rotation, i.e. that they are side by side and the bullet strikes between more or less in line with the block's center of mass.

    In which case the ballistic pendulum reduces merely to solution along the lines you have outlined. Momentum is conserved and then the Kinetic Energy will become potential and hence height can be determined.

    As to using the template you should enter text between the headings that are automatically supplied.
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