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: Dynamics - Collision Question

  1. Mar 23, 2008 #1
    1. The problem statement, all variables and given/known data

    http://img138.imageshack.us/img138/9476/phyus3mu7.jpg [Broken]​

    2. Relevant equations

    Conservation of momentum
    Conservation of kinetic energy

    3. The attempt at a solution

    I really have no idea. I dont know the velocity of A before the collision and i dont know the speed of A after the collision. So i cant use the conservation of momentum. I also know its not an elastic collision because im told "the restitution is e=0.5". Should i have been told some kind of equation to allow me to work this out or am i missing something?

    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Mar 23, 2008 #2

    Shooting Star

    User Avatar
    Homework Helper

    Both. In short, (don't feel like using latex), if co-eff of restitution is e=1/2, then,

    relative velo after collision = vb - va_f = (1/2)va_i = relative velo before collision.

    Using consvn of mom, ma*va_i = ma*va_2 + mb*vb.

    All velos are +ve to the RHS. Solve for va_f. The masses and vb are given.
    Last edited: Mar 23, 2008
  4. Mar 23, 2008 #3
    Hi shooting star, i really appreciate the help but im still a little stuck, would you mind answering a few more questions for me?

    Is this correct:
    ma = Mass A = 1kg
    va_i = Velocity of A initially = Unknown
    va_2 = Final velocity of A? = Unknown
    mb = Mass of B = 10kg
    vb = Final Velocity of B? = 0.876m/s


    Im also a little confused by this equation. Does that mean that in effect relative velo after collision is equal to -relative velo before collision?

    Or was it meant to read like this:

    Finally, am i meant to rearrange this equation
    to make vb the subject and then factor it into the conservation of momentum equation you gave me? I only ask because i still have two unknowns at this point, va_i and va_f.

    Again thank you for the help, sorry i have to ask more questions.
  5. Mar 23, 2008 #4
    The coefficient of restitution between to objects (1 and 2) is given by
    [tex]C_r = \frac{V_{2f}-V_{1f}}{V_{1i}-V_{2i}}[/tex]

    If A is object 1 and B is object 2, then this simplifies to:
    [tex].5 = \frac{V_{Bf}-V_{Af}}{V_{Ai}}[/tex]
    Since the initial velocity of B is zero.

    And then you have
    [tex]M_{A}V_{Ai} = M_{A}V_{Af}+M_{B}V_{Bf}[/tex]
    I think Shooting Star may have meant solve for [tex]V_{Af}[/tex]. Looks like you can do it with those two equations.
  6. Mar 23, 2008 #5

    Shooting Star

    User Avatar
    Homework Helper

    Last edited: Mar 24, 2008
  7. Mar 23, 2008 #6
    Thank you both for the help. For some reason or another i wasnt given these equations. Ill give it a go tomorrow and see how it goes.
  8. Mar 23, 2008 #7
    That worked :D
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook