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: Elastic collision in one dimension

  1. Jul 28, 2009 #1
    1. The problem statement, all variables and given/known data
    A Pendulum weighs .5kg and has a string length of 70cm swings from a horizontal position downwards to hit a block that weighs 2.5 kg and is on a frictionless plain. Calculate the speed of both the ball and the block after the elastic collision.

    2. Relevant equations
    conservation of momentum
    m1V= m1V1+m2V2
    conservation of kinetic energy

    3. The attempt at a solution

    I used the first equation to solve for the initial velocity of the pendulum, which is 3.7m/s.
    From here comes the elastic collision part.
    I try to use both conseravation equations and substitute for one of the variables by solving for either V1 or V2 in either equation and thus substituting. However, when i try to solve it that way it ends up canceling out.
    I know there is an equation for one dimension elastic collisions, but my professor says we can't use it unless we write the derivation of it, and that is a tedious thing to do and is rather lengthy.

    Please help, ask questions if things are unclear
  2. jcsd
  3. Jul 28, 2009 #2


    User Avatar
    Homework Helper

    You can rewrite eq.(1) as
    m1v - m1v1= m2v2...(3)
    Similarly you can rewrtie eq.(2) as
    m1v^2 - m1v^21= m2v^22..(4)
    From 3 and 4 you can get
    (v - v1)/v^2 - v1^2= v[/SUB]2[/SUB]/v[/SUB]2[/SUB]^2
    After simplification you get
    (v + v1) = v2
    Substitute the value of v2 in equation (1) and solve for v1.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook