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Homework Help: Impulse,momentum and direction

  1. Dec 17, 2011 #1
    1. The problem statement, all variables and given/known data
    A cricket bat strikes a ball of mass 0.16 kg travelling towards it. The ball initially hits the bat at a speed of 25m/s and returns along the same path with the same soeed. Time of impact is 0.003s
    a)Determine change in momentum of the ball
    b)Determine FOrce exterted on the ball

    2. Relevant equations
    a) momentum=mass*velocity

    3. The attempt at a solution
    What im trying to ask is, given that v is negative to due to the ball returns in opposite direction, and u is positive then, shouldnt the awnser be: (.16)(-25)-(.16)(25)= -8Ns.

    However, Im confused when the answer shows that it is 8Ns. Why is it so? Does questions like this account for magnitude only? o_O
    Is it still correct if i put -8 as the awnser?
  2. jcsd
  3. Dec 17, 2011 #2
    Impulse is a vector quanity which is why you have you get the answer -8Ns as opposed to +8Ns.

    You could just define the initial velocity, u to be in the negative direction and the final velocity, v to be in the positive direction. That way you would get 8Ns.

    If the question asks for the magnitude of impulse then you take the positive value. As long as you clearly define which direction is positive and which is negative, your answer is correct.
  4. Dec 17, 2011 #3
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