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Penguin and slide. Check my answers? I want to be prepared for the test

  1. Feb 13, 2009 #1
    1. The problem statement, all variables and given/known data
    A sled weighing 60 N is pulled horizontally across snow so that the coefficient of kinetic friction between sled and snow is .100. A penguin of 70 N rides on the sled
    a) The penguin digs claws into sled, what value of F do you need for the sled and penguin to move at constant speed
    b) The penguin releases grip on the sled, coefficient of static friction between penguin and sled is .700, find the maximum horizontal force F that can be exerted on the sled before the penguin starts sliding off

    2. Relevant equations
    F=ma
    Newton's 2nd and 3rd? law

    3. The attempt at a solution
    a) constant speed implied a=0
    F=ma -> F=0
    FN=Fg, FN=mg
    Fapplied=friction
    Fapplied=(.100)FN=.100*(60+70)=13N to move at constant speed

    b) actually only 3 forces acting on the penguin FN Fg static friction
    static friction = .700*mg
    =.700(70)=49 N = max static friction threshold. So a force thats > 49 N needs to be exerted on the slide before the penguin falls off?

    I am assuming kinetic friction has no role in this problem and that since acceleration is not asked for I don't have to set 49 to ma to find the max acceleration before it slides off and everything.
     
  2. jcsd
  3. Feb 13, 2009 #2

    Doc Al

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    Staff: Mentor

    Good.

    You found the maximum static friction force that the sled can exert on the penguin. But what you need to find is the maximum force F you can exert on the sled before the penguin starts to slide off.

    That's exactly what you have to do! :wink: (Just because they don't ask for the acceleration, doesn't mean you don't need to solve for it as a step towards the solution.)
     
  4. Feb 14, 2009 #3
    ok but when i set 49 to ma, i am kind of confused as to which mass to use to find the acceleration. the sled's mass or the total mass?
     
  5. Feb 14, 2009 #4

    Delphi51

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    Homework Helper

    You are doing F = ma for the penguin only, so use the mass of the penguin.
    The maximum force that can be exerted on the penguin by the sleigh is that friction force, thus the limit on the acceleration.
     
  6. Feb 14, 2009 #5
    so it would be 49=(70/9.8)a and then a = 6.86 m/s^2?
    so that is the MAX acceleration this system can have before the penguin slides off right?

    so in order to find the force that I need to apply on the sled to achieve this acceleration, I then use F=ma again? but this time, with the mass of the sled AND the penguin? not so sure but i am thinking about it like this:

    the penguin will stay on the slide as long as the max acceleration is 6.86, so I am applying force on both masses. right?
     
  7. Feb 14, 2009 #6

    Delphi51

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    Homework Helper

    Yes on all points!
     
  8. Feb 14, 2009 #7
    hmm I think I am starting to get forces, gotta get into energy next argh >.<

    but thanks for the help delphi51 and doc al
     
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