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Work on a baseball being caught by a catcher

  1. Nov 27, 2009 #1
    1. The problem statement, all variables and given/known data
    A baseball of mass 250g (0.25kg), pitched with a speed of 40m/s, is caught by a catcher whose glove moves backward 0.25m while stopping the ball.

    a) What is the kinetic energy of the ball? (solved)
    b) How much work did the catcher's glove do on the ball?
    c) What was the average stopping force on the ball?

    Given: mass=0.25kg; speed=40m/s; distance=0.25m

    2. Relevant equations
    Ek = 1/2(m)(v^2)
    Work = F x d

    3. The attempt at a solution
    I am completely stumped, I'm trying for the life of me to figure out how to find the Applied Force on the ball with what I've been given this unit. I'm not sure how to solve for the Force Applied without two of the other variables given for the work equation.

    As for question C, what are they asking for? I'm unsure of what they mean by "stopping force".
     
  2. jcsd
  3. Nov 27, 2009 #2

    PhanthomJay

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    If you are not familiar with the work-energy theorem, use the kinematic equations to solve for the deceleration of the ball, then apply Newton 2.
     
  4. Nov 27, 2009 #3
    Is the theorem Work = Ekfinal - Ekinitial? If the initial kinetic energy = 5J, the final should be 0Jm because you're multiplying by 0 m/s. This gives me -5J which throws me off because I don't know if I should be taking the absolute value or not.
     
  5. Nov 27, 2009 #4

    PhanthomJay

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    Work can be either positive or negative, depending on the direction of the force and the displacement.
     
  6. Nov 27, 2009 #5
    Ah I understand, so when they ask for me to find "the average stopping force" are they asking me to re-arrange the W=F x d formula to solve for the force?

    I'm really sorry about all the stupid questions I'm just trying to wrap my head around everything.
     
  7. Nov 27, 2009 #6

    PhanthomJay

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    Yes, but remember it's W=F x d x cos theta, where theta is the angle betwen the force and displacemnt vectors. What does the minus sign imply?
     
  8. Nov 27, 2009 #7
    Does it imply a change in the displacement vector?
     
  9. Nov 28, 2009 #8

    PhanthomJay

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    Imagine that I am the pitcher and you are the catcher. When you catch my pitched baseball, your glove moves (displaces) backward. Which way does the stopping force of your glove on the baseball act: forwards, or backwards? One of the keys to mastering Physics is mastering the plus and minus sign.
     
  10. Nov 28, 2009 #9
    I would say the stopping force acts in the opposite direction that the ball is thrown, whilst the glove moves forewards in the same direction as the ball. Correct? Or have i gotten myself all backwards haha.
     
  11. Nov 28, 2009 #10

    PhanthomJay

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    You've got it right.:biggrin:
    That's why the work is negative.
     
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