1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Bullet Through Pendulum Bob-Inelastic Collision

  1. Nov 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A bullet of mass m and speed v passes completely through a pendulum bob of mass
    M. The bullet emerges on the other side of the pendulum bob with half its original
    speed. Assume that the pendulum bob is suspended by a stiff rod of length L and
    negligible mass. What is the minimum value of v such that the pendulum bob will
    barely swing through a complete vertical cycle

    2. Relevant equations

    m1v1i2 + m2v2i2 = m1v1f2 + m2v2f2


    k = 1/2mv2

    p = mgh

    3. The attempt at a solution
    initial kinetic energy of bullet = potential energy of bob @ max height + final kinetic energy of bullet

    1/2mv2 = Mg2L + 1/2 m(1/2v)2
    1/2mv2 - 1/8 mv2 = 2MgL
    v2 = (16MgL)/(3m)
    v = 4[(MgL)/(3m)]^(1/2)

    Answer is [4M(gL)^(1/2)]/m

    The key sets
    1/2MVb2 = Mg2L

    if Vb = velocity of the bob, then at max height the bob is not moving hence velocity is 0 and all of the energy that the bob carried at the bottom is now potential energy.

    and then solves via the equation:
    mv = MVb + mv/2

    initial momentum = momentum of bob + momentum of bullet after going through bob

    If everything that I have said about the professors method is true, then I feel like I am beginning to understand why the professors way works, but why doesn't my method come to the same conclusion.
    Last edited: Nov 1, 2013
  2. jcsd
  3. Nov 1, 2013 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Hello, and welcome to PF!

    Is the collision of the bullet with the bob elastic or inelastic?
  4. Nov 1, 2013 #3
    Looking back on the pdf, it is inelastic because kinetic energy is being transformed into potential energy.
    But why can't we say that
    initial kinetic energy = sum of final energies?
  5. Nov 1, 2013 #4


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    What type of energy does an inelastic collision produce? Did you include that type of energy in your sum of final energies?
  6. Nov 2, 2013 #5
    An inelastic collision will take kinetic energy and transform it into some other types of energy like potential or thermal. (from my textbook). Am I not allowed to say that the initial kinetic energy of the bullet is equal to the potential energy imparted on the bob + what is left of the kinetic energy of the bullet?

    KA + KB = K'A + K'B + thermal & other forms of energy

    initially, the bullet has all of the energy of the system, so KB would be 0, after the bullet will have 1/2 the velocity so it will have 1/4 the kinetic energy and the kinetic energy of the bob should be 0 because it has no velocity at the top of the loop. So potential energy of the bob must be 3/4 KA.
    I feel like something in my logic isn't correct though, as it gives me an entirely different answer.
    Last edited: Nov 2, 2013
  7. Nov 2, 2013 #6
    You simply can't conserve energy before and after the collision here. Not all of the kinetic energy lost by the bullet will be converted into kinetic energy in the bob. There will be some losses.
  8. Nov 2, 2013 #7


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    This equation is correct. Note that you need to include the thermal energy on the right side. But you don't know the amount of thermal energy created in the inelastic collision, so this equation has too many unknowns.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted