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Boat tugging a whale

  1. Jul 16, 2011 #1

    Femme_physics

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    Am getting stuck with 2 unknowns


    1. The problem statement, all variables and given/known data


    http://img40.imageshack.us/img40/5883/whaltboat.jpg [Broken]

    The 5.5-Mg humpback whale is stuck on the shore due to changes in the tide. In an effort to rescue the whale, a 12-Mg tugboat is used to pull it free using an inextensible rope tied to its tail. To overcome the frictional force of the sand on the whale, the tug backs up so that the rope becomes slack and then the tug proceeds forward at 3 m/s.
    If the tug then turns the engines off, determine the average frictional force F on the whale if sliding occurs for 1.5 s before the tug stops after the rope becomes taut. Also, what is the average force on the rope during the tow?


    3. The attempt at a solution
    As u can see, I get stuck with 1 equation and two unknowns. I was able to solve for N and a (hopefully I did it correctly) though.

    http://img121.imageshack.us/img121/3028/19084315.jpg [Broken]



    http://img339.imageshack.us/img339/7913/16323840.jpg [Broken]
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jul 16, 2011 #2
    to find Tension (T)
    tension retards the ship
    so
    T=mass of ship * acc of ship(a)
    T= 12000*2
    = 24000 N

    now u can find Friction usin

    m*a = T -F

    F = T - m*a
    =24000 - 11000
    =13000 N
     
  4. Jul 16, 2011 #3

    I like Serena

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    Hey Fp! :smile:

    Can you make an FBD of the ship?

    Perhaps that will give you an extra equation?
     
  5. Jul 16, 2011 #4

    Femme_physics

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    Last edited by a moderator: May 5, 2017
  6. Jul 16, 2011 #5

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    Good! :)

    Do you have enough information now to find the friction force?
     
  7. Jul 16, 2011 #6

    Femme_physics

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    Last edited by a moderator: May 5, 2017
  8. Jul 16, 2011 #7

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    What?
    No victory dance? :confused:
     
  9. Jul 16, 2011 #8

    Femme_physics

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    I got it?!?? :biggrin:

    w000t! *victory dance!!!!*


    Wait, let's do it with calculus now!!!!

    Give me pointers, I'm sooooooooooo ready!
     
  10. Jul 16, 2011 #9

    Femme_physics

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    Hmm wait according to the manual

    F = 24[kN]
    T = 24 [kN]
     
  11. Jul 16, 2011 #10

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    Hmm, if F equals T, then....
    The resultant force on the whale is zero...
    That doesn't sound right. :confused:

    I know, I think they made a typo (a copy+paste error)! :smile:



    Ah, but then we would need Captain Calculus! Rawwwr!
    He got points and angles!

    [PLAIN]http://www.cantonschools.org/~lforastiere/00F5BEEB-0075833E.0/Captain%20Calculus.jpg [Broken]
     
    Last edited by a moderator: May 5, 2017
  12. Jul 16, 2011 #11
    hmmm......sounds right

    i got it now: *ques asks for average force....average acceleration is zero so is average net force*

    i got the same answer using calculus
     
  13. Jul 16, 2011 #12

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  14. Jul 16, 2011 #13

    I like Serena

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    Nope.
    Average acceleration is not zero, since it says in the problem: "sliding occurs for 1.5 s before the tug stops".
     
  15. Jul 16, 2011 #14
    but it stops after sliding 1.5 s...that means it retards which cancels the intiall acceleration and average becomes zero
     
  16. Jul 16, 2011 #15

    Femme_physics

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    Oh now you're just patronizing. Humphffh!! :grumpy: :tongue:

    So they're mistaken?

    LOL where'd you get that from?!? :rofl:

    Captain Calculus. I'm gonna use that haha!
     
  17. Jul 16, 2011 #16

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    The acceleration is never negative, so it can't cancel, nor can its average become zero.


    I believe that they are mistaken.


    I was googling for funny pointers on calculus when I found the book.
    Looking at it I found it *mildly enjoyable* :D


    And errrr.... I don't know how to do this problem with calculus....
     
    Last edited: Jul 16, 2011
  18. Jul 16, 2011 #17
    check this
     
  19. Jul 16, 2011 #18

    I like Serena

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    Check what?
     
  20. Jul 16, 2011 #19
    the meanig of the sentence
     
  21. Jul 16, 2011 #20

    Femme_physics

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    Really? But I thought you studied physics with calculus since you took it at the university
     
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