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Conservation of Momentum of a snow ball

  1. Feb 3, 2014 #1
    A 35-kg girl is standing near and to the left of a 43-kg boy on the frictionless surface of a frozen pond. The boy tosses a 0.75 kg ice ball to the girl with a horizontal speed of 6.2 m/s. What are the velocities of the boy and the girl immediately after the girl catches the ball?

    I don't know how to approach this

    (1) I have mass1 (girl) and mass2 (boy) , so that kind of tells me that I have to use conservation of momentum theorem.

    m1v1 + m2v2 = (m1 + m2) v (is this "v" 1 or 2???)

    is this ok? if so, what else??? I dont know what velocity
     
  2. jcsd
  3. Feb 3, 2014 #2

    CWatters

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    Could you solve it if the problem was in two parts? The first part being...

    1) A 43-kg boy on the frictionless surface of a frozen pond tosses a 0.75 kg ice ball with a horizontal speed of 6.2 m/s. What is the velocity of the boy after he throws it?
     
  4. Feb 3, 2014 #3
    m1v1 + m2v2 = (m1+ m2)v

    (35kg)(0) + (.75kg)(6.2 m/s) = ((43kg) + (0.75kg)) V
    (4.65)=(43.75) V
    v=.11m/s

    Now, I am so confused about how to get the girls velocity
     
  5. Feb 3, 2014 #4
    That is incorrect .

    In the first part as suggested by CWatters ,the girl doesn't come in picture .The boy tosses the ice ball in air towards left .The girl hasn't caught it yet .Forget the girl .Think about ice ball and the boy .

    Initial Momentum = Final momentum

    Initial Momentum = 0

    0 = m1v1+m2v2

    Here m1 is ice ball and m2 is boy . v1 is velocity of the ice ball just after it leaves the boy and is in air .v2 is the velocity of the boy after it throws the ball .Take care of the signs .
     
  6. Feb 5, 2014 #5

    CWatters

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    What Tanya said. Ignore the girl and just apply conservation of momentum to the boy and ice ball. You don't care what happens to the ice ball after it's thrown, if it just landed on the ground it wouldn't effect what happens to the boy. eg the mass of the girl cannot affect what happens to the boy so her mass won't feature in the equations at this stage.

    When you have solved the boy an ice ball part of the problem only then turn your attention to the girl and ice ball and forget about the boy.
     
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