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Homework Help: Bullet hitting a block of wood

  1. Aug 1, 2010 #1
    1. The problem statement, all variables and given/known data
    A bullet leaves the muzzle of a rifle at an unknown velocity and strikes a wooden block that is suspended by a piece of string.
    If the block swings backwards to make an angle of 40 degrees, at what velocity must have the bullet been travelling when it struck the block?
    mass of the bullet and the wood are, respectively, m=0.05 kg and M=2 kg, and the length of the string is L=1 m. Theta = 40 degrees
    [PLAIN]http://img810.imageshack.us/img810/5295/bullet.jpg [Broken]

    2. Relevant equations

    3. The attempt at a solution
    The block is given kinetic energy equal to the kinetic energy the bullet had.
    Since the block experiences no friction, all of this kinetic energy is used up by moving against gravity.
    s and f can be determined and so the energy can be calculated
    with f x s = energy
    Then use (1/2)mv^2 = energy to calculate the speed of the bullet

    The problem is that I'm unsure how to calculate s. How can this be done? (s = vertical displacement of the block)
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 1, 2010 #2


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    Science Advisor
    Homework Helper
    Gold Member

    Incorrect. See below.
    The block experiences no friction only after the bullet comes to rest inside it. While the bullet is slowing down inside the block there is a lot of friction. So some of the initial kinetic energy of the bullet goes into heat generated by friction.
    Can you find how fast the bullet+block system is moving after the bullet becomes fully embedded?
  4. Aug 1, 2010 #3
    @ kuruman, i think at a high school/ introductory physics level we're allowed to assume that energy is conserved within the bullet-block system.

    The solution is simple. It is a mere calculation using the conservation of energy.

    0.5*m*v^2 = m*g*h

    Theta is given to be 40 degrees. So in order to calculate the rise in the height of the block,
    Lcos40 = h
    g is a constant. The rest are all given.
    working that out you should get
    V = 24.5 m/s ( 3sf )
  5. Aug 1, 2010 #4

    Doc Al

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    Staff: Mentor

    Absolutely not! You've missed a key point of the exercise.

    Simple, but incorrect. Listen to kuruman!

    Let the OP do their own work please.
  6. Aug 1, 2010 #5
    Ry122, do you know what the answer to the question is?
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