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Homework Help: Momentum unknow speed

  1. Jul 9, 2011 #1
    Sorry for another momentum question I want to be sure.

    1. The problem statement, all variables and given/known data
    A ball who is coming at 180 degree is hit by a baseball bat at 45 degree in the opposite direction. The final speed of the ball and the angle is unknown and this is want we search.

    N*S=kg *m/s
    I=5 N*S= 5 kg*m/s
    Initial ball speed Vo=-30m\s
    Bat speed =Vb
    Masse M=0.150 Kg

    2. Relevant equations
    So I suppose that since the ball is hit at 45 degree
    the impulsion will be
    5 kg*m/s*cos(45)=the horizontal vector and 5 kg*m/s *cos(45)=the vertical vector

    3. The attempt at a solution
    5 kg*m/s*cos(45)= 3.5 I
    5 kg*m/*sin(45)= 3.5 J
    (3.5 I + 3.5J) kg*m/s= 0.150kg(Vb- ( - 30I) )m\s
    (23.3 I + 23.3 J )m/s= (Vb+30I) m\s
    (-6.7 I + 23.3 J ) m/s = Vb Not in the opposite direction ????
    For the final speed = -6.7 I+28J -30I= -36.7I +28J
    hmm the baseball bat have hit the ball but not and the right direction, something most be wrong
    thank you

    Attached Files:

    Last edited: Jul 9, 2011
  2. jcsd
  3. Jul 9, 2011 #2

    Andrew Mason

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    Homework Helper

    Conservation of momentum of the bat and ball does not apply here. The reason is that the bat is not a free body. It is connected to the batter who is, in turn, connected to the earth.

  4. Jul 9, 2011 #3
    Hmm any ideas how to solve ?
    I don't understand Andrew, there are some exemples like this one in my book, but they give the final speed and the initial speed ( the momentum is to find). What is the difference ?
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