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Homework Help: Cardboard Contact

  1. Dec 6, 2011 #1
    Hi this is a problem from my midterm at the beginning of the quarter and i have a final coming up so i need to know the previous midterm material but i still dont understand this one question.

    Dave suddenly drops from the wire and lands at a velocity of 2m/s on some cardboard boxes that contract to absorb the force of his fall. If the cardboard boxes exerts a force of 350N on Dave who weighs 70kg, how much do the cardboard boxes contract?


    the equation i would use is v^2=vo^2 - 2adelta y

    but in her solutions she shoes delta x. Why is it delta x if the cardboard is going to be contracting in the y direction?

    If someone can help me start off this problem i will so grateful!
     
  2. jcsd
  3. Dec 6, 2011 #2
    Off the top of my mind, I think her equations are supposed to be general. You can substitute delta y for delta x. If you continue on with that assumption, does it get you the right answer?
     
  4. Dec 6, 2011 #3
    yeah thats what i thought too but there are two equations, one for x and one for y and they differ.

    v^2 = Vo^2+2adelta x

    v^2 = Vo^2-2gdelta y

    in x you add, in y you subtract
     
  5. Dec 6, 2011 #4
    That's general kinematics. You can't just plug in the exact numbers for the exact equations and hope for the best. At what rate is David accelerating when he jumps off? I assume that his initial vertical v is zero. If you figure that out, you can find the unknown y.
     
  6. Dec 6, 2011 #5
    nvm i think youre right. It was just difficult for me to apply the force and the mass to the equation. I thought gravity would be used instead.
     
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