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Question on Net Force

  1. Sep 25, 2014 #1
    1. The problem statement, all variables and given/known data
    Problem #7
    (Due 9/26 Friday by 8:00 a.m.)
    Hydro, who has a mass of 45.1 kg, falls straight down 4.67 m to the water below.
    a. If Hydro comes to rest .543 seconds after hitting the water, what was the net force exerted on Hydro over this time?


    2. Relevant equations



    3. The attempt at a solution
    Looked in the book, read the sections the question pertains to, and am still lost. Any assistance with how to solve this would be appreciated.
     
  2. jcsd
  3. Sep 25, 2014 #2
    You can use impulse to solve this.
     
  4. Sep 25, 2014 #3
    (4.67/.543) *45.1?
     
  5. Sep 25, 2014 #4

    Doc Al

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    Staff: Mentor

    No.

    What's the momentum of Hydro as he first touches the water?
     
  6. Sep 25, 2014 #5
    4.67/.543 so 8.60?
     
  7. Sep 25, 2014 #6
    M/s
     
  8. Sep 25, 2014 #7

    Doc Al

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    Staff: Mentor

    Don't just give numbers--show how you used the basic formulas.

    In any case, no. Answer this: If something falls a distance X, how fast is it going at that point?
     
  9. Sep 25, 2014 #8
    Acceleration of gravity correct?
     
  10. Sep 25, 2014 #9

    Doc Al

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    The acceleration of a falling object is g, the acceleration due to gravity. But what's the speed of the object after it falls?
     
  11. Sep 25, 2014 #10
    I'm struggling mightily.
     
  12. Sep 25, 2014 #11
    What may seem obvious is just not registering for me here
     
  13. Sep 25, 2014 #12
    speed = distance / time
    so
    4.67m / 0.543s
    but how do you get the acceleration from that?
     
    Last edited: Sep 25, 2014
  14. Sep 25, 2014 #13


    I'm also working on this question for homework.
    For the velocity vs. time graph, does your constant negative line start at some velocity above zero and stop at zero? Or does it go past zero into a negative, or does it start at zero and go down?
    Wasn't very good at drawing these for the test either...
     
  15. Sep 25, 2014 #14
    Divide by time again
     
  16. Sep 25, 2014 #15
    I wasn't either. Go to umd? I believe it goes into a negative but I'm confused with the class so far
     
  17. Sep 26, 2014 #16

    Doc Al

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    That will give you average speed.

    You are dividing the distance it falls (before it hits the water) by the time it takes to come to rest (after it hits the water). That won't help.

    Separate this problem into two parts: (1) How fast is it moving when it hits the water? (2) What net force does it have after it hits the water and is slowing down?

    Solve each part separately.

    The acceleration due to gravity is a constant. Look it up! (It's usually called g.)
     
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