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General Physics- Frictional Forces on Objects

  1. Aug 7, 2012 #1
    1. The problem statement, all variables and given/known data
    A block of mass 3 kg, which has an initial
    speed of 9 m/s at time t = 0, slides on a
    horizontal surface.
    Find the magnitude of the work that must
    be done on the block to bring it to rest.
    Answer in units of J
    I already got 121.5 for this problem

    If a constant friction force of magnitude
    4 Newtons is exerted on the block by the surface, find the magnitude of the acceleration of
    the block.
    Answer in units of m/s^2

    How far does the block slide before it comes
    to rest?
    Answer in units of m



    A team of dogs drags a 94.8 kg sled 2.63 km
    over a horizontal surface at a constant speed.
    The coefficient of friction between the sled
    and the snow is 0.228.
    The acceleration of gravity is 9.8 m/s^2
    Find the work done by the dogs.
    Answer in units of kJ

    Find the energy lost due to friction.
    Answer in units of kJ

    2. Relevant equations

    F = μmg

    W = Fd

    W = μmgd





    3. The attempt at a solution

    I got the first third of problem one, but I don't know how to account for the frictional force.

    For the second problem, I don't know how to account for the force of gravity.

    Thanks anyone,
    Physics-Pure
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Aug 7, 2012 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Physics-Pure! Welcome to PF! :smile:

    (try using the X2 button just above the Reply box :wink:)
    yup! :biggrin:
    you're told what the frictional force is, so just use F = ma :wink:
    it comes in your own equation F = µmg :wink:
     
  4. Aug 7, 2012 #3
    Thank you very much :D

    Now, what about part three of question one, and part two of question two?
     
  5. Aug 7, 2012 #4

    tiny-tim

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    come on, you know the rules :wink:

    show us how far you've got! :smile:
     
  6. Aug 7, 2012 #5
    I don't know how to begin them. D:
     
  7. Aug 7, 2012 #6

    berkeman

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    Staff: Mentor

    Please summarize what you have so far, and what you do not understand. We certainly will not do your work for you, but perhaps if you summarize what is confusing you, we can offer a hint...
     
  8. Aug 7, 2012 #7
    Well the problem is, is that I'm skipping honors physics at my high school and taking AP Physics as a Freshman. However, I am required to complete honors physics review work. Therefore, I have no physics knowledge whatsoever, and do not know how to begin the third part of question 1 and the second part of question two.
     
  9. Aug 7, 2012 #8

    berkeman

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    Staff: Mentor

    That doesn't help. Please summarize the questions that you are having trouble with. It's not clear where the question numbers are...
     
  10. Aug 7, 2012 #9
    Those are the two sections to which I am referring.
     
  11. Aug 7, 2012 #10

    berkeman

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    Staff: Mentor

    If I understand where you are, you have the energy required to bring it to rest, yes? What is the equation relating work (energy), force and distance?
     
  12. Aug 7, 2012 #11
    W = Fd. I have the answers to parts one and two of the first question, and part one of the second question.
     
  13. Aug 7, 2012 #12

    berkeman

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    That's the right equation. You already calculated the work, and you know the force...
     
  14. Aug 7, 2012 #13
    Alright, thank you. Now moving onto the second part of question two?
     
  15. Aug 7, 2012 #14

    berkeman

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    Staff: Mentor

    Yes, please do. Re-read the full question, and see if you can figure it out for yourself. Think about where energies are coming from and going...
     
  16. Aug 7, 2012 #15
    I know that the work done by the dogs was 557.089 KJ, but I don't know how to find the energy lost due to the friction coefficient.
     
  17. Aug 7, 2012 #16

    berkeman

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    Staff: Mentor

    Was the sled accerating or moving at a constant speed? If there is no acceleration, what did all that dog work go into doing?
     
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