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What wrong with this problem?

  1. Sep 21, 2009 #1
    What wrong with this problem???

    A ball is traveling horizontally over a volleyball net when a player "spikes" it, driving it straight down to the ground. The ball's mass is 0.24 kg, its speed before being hit is 6.4 m/s and its speed immediately after the spike is 21 m/s. What is the magnitude of the impulse from the spike?

    J=(.24kg)(21m/s)-(.24kg)(6.4m/s)=3.5

    this is wrong, whats wrong
     
  2. jcsd
  3. Sep 22, 2009 #2
    Re: What wrong with this problem???

    Still need some input on this please?
     
  4. Sep 22, 2009 #3

    Doc Al

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    Staff: Mentor

    Re: What wrong with this problem???

    Impulse and momentum are vectors--you must take direction into account. Subtract them as vectors, not just numbers.
     
  5. Sep 22, 2009 #4
    Re: What wrong with this problem???

    This probably sounds stupid but what do u mean by subtract them as vectors.

    I know know what vectors are but...
     
  6. Sep 22, 2009 #5

    Doc Al

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    Staff: Mentor

    Re: What wrong with this problem???

    You just subtracted them as numbers--which means you treated them as if they pointed in the same direction.

    To subtract vectors, draw yourself a diagram. The two vectors are perpendicular to each other. Or you can find their components and subtract them.

    Hint: Subtracting is the inverse of adding. How would you add two perpendicular vectors?
     
  7. Sep 22, 2009 #6
    Re: What wrong with this problem???

    again this sounds dumb, but would add these as vectors give you

    (.48kg, 27.4m/s)
     
  8. Sep 23, 2009 #7

    Doc Al

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    Staff: Mentor

    Re: What wrong with this problem???

    No, that doesn't quite make sense.

    I'll start you off. Lets call horizontal and vertical the x and y axes (like usual).

    The initial momentum is:
    mv = (.24 kg)*(6.4 m/s) = 1.54 kg-m/s in the + x direction
    Expressed as a vector using components, it would be:
    (1.54, 0)

    The final momentum is
    mv = (.24 kg)*(21 m/s) = 5.04 kg-m/s in the - y direction
    Expressed as a vector using components, it would be:
    (0, -5.04)

    Now subtract those two momentum vectors and find the magnitude of the impulse.
     
  9. Sep 23, 2009 #8
    Re: What wrong with this problem???

    Alright got it
     
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