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: Collision question - Simple one

  1. Nov 8, 2011 #1
    1. The problem statement, all variables and given/known data

    Something I am doing wrong?

    "A 65kg boy and his 40kg sister both wearing roller blades, face each other at rest. The girl pushes the boy hard, sending him backwards with a velocity of 2.90 m/s toward the west. Ignore friction.

    What speed does the girl go?"

    I cam up with m1(v1i - v1f) = m2(v2f - v2i)

    Solved for v2f and got 3.9 m/s

    But the correct answer is 4.71 m/s

    ?? Any help?



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 8, 2011 #2

    DaveC426913

    User Avatar
    Gold Member

    m1 * v1 = m2 * v2
    where
    m1 = 65
    v1 = 2.9
    m2 = 40
    v2 = ?
     
  4. Nov 8, 2011 #3
    what the heck, I am still getting 3.9 :(

    im going (65)(2.4)=(40)(x) solve for x, which = 3.9
     
  5. Nov 9, 2011 #4
    You should have a 2.9 instead of your 2.4 :)
     
  6. Nov 9, 2011 #5

    DaveC426913

    User Avatar
    Gold Member

    I imagine he's beating his head on his keyboard right about now...:tongue2:
     
  7. Nov 9, 2011 #6
    Haha, Dave, yes I was!

    Thanks, I guess my 4's look way too much like 9's :)
     
  8. Nov 9, 2011 #7

    DaveC426913

    User Avatar
    Gold Member

    on ... your ... keyboard ? :bugeye:
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook