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: Find angular velocity

  1. Nov 21, 2006 #1
    The problem is problem 6 at


    A long thin rod of mass M and length L with two balls of mass M1 (same mass for both) attached is allowed to rotate about the horizontal axis shown. The bar is initially stationary. It is then hig with a piece of putty of mass M2 and speed v which sticks to one of the M1's.

    a) Find the angular velocity of the system after the collision. The correct answer should be 6M2*v/ (6M1L + 3M2L + ML)

    b) What angle will the sytem rotate through before coming to a stop? Assume that it must be between 180 and 270 degrees.

    For part a,

    Am I suppose to use the conservation of angular momentum.

    IW = I_f*W_f
    (1/12)ML^2 *w= (1/12)(M1+M2)*L^2*W_f

    I am stuck though since I don't know how to account for v.

    For part b,

    The answer is 180 + arcsin(V^2/gL)

    I don't know how to get to v^2/gL.
  2. jcsd
  3. Nov 22, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    The rod with attached masses has a moment of inertia you can calculate and no initial angular momentum. You account for v by looking up the fundamental definition of angular momentum (for a moving particle; not the derived expression involving moments of inertia for a rigid assumbly of particles). It involves the mass and velocity and what else?
  4. Nov 22, 2006 #3
    Angular momentum = m*r*v

    I know that I for a rod is (1/12)ML^2

    so L = Iw


    L = (1/12)ML^2*v^2/L^2
    L = (1/12)MV^2*L^2

    r = 2L

    If I have

    IW = I_f*W_f

    W_f = I/I_f * W

    W_f = (1/12)ML^2/(I_f) * W

    I am stuck. I don't know what to do from here.
  5. Nov 22, 2006 #4


    User Avatar
    Science Advisor
    Homework Helper

    You need to fix the relationships between r and L and between v and ω. Be careful to distinguish the initial particle velociy from the velocity after the collision, and be careful about the lengths involved in the problem.
  6. Nov 22, 2006 #5
    I made some mistakes. L = 2r, r = (1/2)L

    w^2=v^2/r^2 or v^2/(1/2*L)^2 = 4v^2/L

    Is M in this case 2M1 since we have two masses in the system before collision. Should the mass be 2M1 + M2 after collision?
  7. Nov 22, 2006 #6


    User Avatar
    Homework Helper

    For conservation of the angular momentum of the interacting system:




    [tex]\frac{l}{2}p_{putty} = \left(I_{rod} + I_{m_1m_2} + I_{m_1}\right) \omega[/tex]
  8. Nov 22, 2006 #7
    Thanks. For part b, I still don't know where v^2/gl come from. G is an acceleration due to gravity, l is a length, and v^2 is m^2/s^2. After division, v^2/gl is just a number with no unit. However, I don't know where the term come from.
  9. Nov 22, 2006 #8


    User Avatar
    Homework Helper

    Try and approach it along these lines

    [tex]W_{torques} = \Delta K[/tex]

    The system experiences two torques, [tex]\Gamma _1,\ \Gamma_{12}[/tex] which is from the weights of m1 and (m1 + m2). The torques will change as the system rotates so you need to integrate to find the work done by these.

    The final kinetic energies are zero. So we are left with only the initial rotational kinetic energies of the three components of the system.
    Last edited: Nov 22, 2006
  10. Nov 22, 2006 #9


    User Avatar
    Homework Helper

    Note that from part a that

    [tex]\omega _i = \frac{pl}{2 I_s}[/tex]

    where [tex]I_s[/tex] is the moment of inertia of the system

    also note that the change in kinetic energy of the system will be

    [tex]\Delta K = -\frac{1}{2} I_s {\omega _i}^2[/tex]
    Last edited: Nov 22, 2006
  11. Nov 22, 2006 #10


    User Avatar
    Homework Helper

    Changed previous post.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook