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: Problem dealing with elastic collisions.

  1. Nov 8, 2008 #1
    1. The problem statement, all variables and given/known data
    Minnie mouse (mass m=37.5g) has run to the top of a curved frictionless wedge (height H1=1.15m) She slides down the track and makes a perfectly elastic collision with mickey mouse (mass m=69.8 g) who is at rest. Mickey flies off the table (height H2= .993 m) above the floor and minnie rebounds to a height H3 before she eventually falls off the table.
    How far from the edge of the table, X1, does mickey land?
    How high up, H3, does minnie rebound?
    How far from the edge of the table, X2, does minnie land?

    2. Relevant equations
    The equation I thought of using was M1V1i+ M2V2i=M1V1f + M2V2f.
    This is not correct.

    3. The attempt at a solution
    I tried to use the equation above but I do not know the velocity of the system so that made me stop. Is that the equation I use? I do not know where to start on this one because I do not have a starting equation to use.
  2. jcsd
  3. Nov 9, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi dban33! :smile:

    For a collision between two bodies, you need two equations.

    For any collision, one of those equations is conservation of momentum (that's the equation you quoted).

    For an elastic collision (only), the other equation is (instantaneous) conservation of energy.

    Have a go! :smile:
  4. Nov 9, 2008 #3
    Ok good Im sortof on the right path then with the conservation of mometum equation. Is the conservation of energy equation V1i-V2i=-(V1f-V2f).

    Both of these equations involve velocity though and I was not given that in the problem, where do I get that from?
  5. Nov 9, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    Hi dban33! :smile:
    Nooo … that equation's rubbish … burn it! :yuck:

    Conservation of energy is KE + PE = constant.
    You'll get the velocity from the conservation of energy equation (the PE doesn't involve velocity). :wink:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook