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Fish Problem

  1. Sep 24, 2006 #1
    All right, I am doing my physics homework and I just cannot figure this one out. I have been trying for literally over an hour, so I am running out of ideas. Here it is.

    The tension at which a fishing line snaps is commonly called the line's “strength.” What minimum strength is needed for a line that is to stop a salmon of weight 76 N in 13 cm if the fish is initially drifting at 3.1 m/s? Assume a constant deceleration.

    All right, so clearly this has to do with force, which is going to end up being the mass of the fish times its decleration. But because they are asking for strength, I am guessing it a magnitude and not a vector. So, I have tried things such as using the velocity over the meters taken to stop the fish, to get time and then apply that to get the acceleration, but that was clearly wrong as the time changes due to the deceleration. I have done all kinds of things, I need help to figure this one out. Thank you
     
  2. jcsd
  3. Sep 24, 2006 #2

    Doc Al

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    Use your knowledge of kinematics to find the acceleration. (You are given the initial speed and the distance--that's all you need.) Then use Newton's 2nd law to find the force needed to produce that acceleration. (What's the mass of the fish?)
     
  4. Sep 24, 2006 #3
    Thank you so much. I hate those problems that are so simple, yet you overlook. I figured out the answer and it was correct. I used the formula 2a(Change in x) = (final velocity) squared minus (initial velocity) squred. but once again, thank you
     
  5. Oct 20, 2006 #4
    another question about the fish problem

    I was wondering if someone could explain the fish problem in a bit more detail, I can't seem to get it...

    Thanks in advance
     
  6. Oct 20, 2006 #5

    Hootenanny

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    Welcome to the Forums Mike,

    How would you approach the problem yourself? How much do you know about kinematics?
     
  7. Oct 20, 2006 #6
    As far as approaching the problem, im not sure, I was doing a few things with F=ma and replaceing the F with W, but i dont seem to be making a whole lot of progress.
     
  8. Oct 20, 2006 #7

    Hootenanny

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    Okay, do you know any kinematic equations?
     
  9. Oct 20, 2006 #8
    Umm what would be an example of some?
     
  10. Oct 20, 2006 #9

    Hootenanny

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    Doc Al has authored this rather nice thread, as a summary of many basic physics formulae. This post in particular deals with basic kinematic equations.
     
  11. Oct 20, 2006 #10
    Yes I do know some kinematics
     
  12. Oct 20, 2006 #11
    I am familiar with all of those yes
     
  13. Oct 20, 2006 #12

    Hootenanny

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    Make two list, one with variables you know. The other with variable(s) you wish to find. Which formulae are applicable in your case?
     
  14. Oct 20, 2006 #13
    I don't see how the initial speed and the distance can get you the acceleration though.
     
  15. Oct 20, 2006 #14

    Hootenanny

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    You also know the final velocity.
     
  16. Oct 20, 2006 #15
    For this problem the displacement and velocity equation would be needed i think, and Newtons 2nd law F=ma and maybe W=mg
     
  17. Oct 20, 2006 #16
    Ohhh, it would be zero
     
  18. Oct 20, 2006 #17

    Hootenanny

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    Spot on.
    Good.
    Sounds good.
    You may want to rethink this. What can you say about the net vertical force is a fish is floating (or swimming) at the same depth?
     
  19. Oct 20, 2006 #18
    it would zero if the fish was at the same depth?
     
  20. Oct 20, 2006 #19

    Hootenanny

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    Correct, so you don't need to worry about the weight of the fish. However, you do need the weight to calculate the ....
     
  21. Oct 20, 2006 #20
    the mass of the fish?
     
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