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Help with physics homework regarding friction, tension, springs and centrifugal force

  1. Sep 5, 2012 #1
    1. The problem statement, all variables and given/known data
    The first problem I am having trouble with is as follows:

    A child pushes a 40kg sled across the ice at a constant speed. If μk = 0.05 calculate the force applied to the sled.


    2. Relevant equations
    I know that f=[itex]\mu[/itex]FN



    3. The attempt at a solution

    I know that the free body diagram would consist of a point with a direction in the x coordinate, but I have no idea how to go about this problem. Math and physics is not my strong subject; this is my first ever physics class
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 5, 2012 #2

    CAF123

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    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Have you tried drawing the free body diagram? The sled is moving at constant speed so the net force acting on it is zero.
     
  4. Sep 5, 2012 #3
    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Here is what I came up with:

    Ʃy=FN-mg=0. Therefore, FN=mg=(40kg)*(9.8 m/s2)=392N.

    Ʃx=F-f=0. Therefore, F=f, which equals [itex]\mu[/itex]s*FN, which equals (0.05)*(392N), giving me an answer of 19.6N. Did I do this correctly?
     
  5. Sep 5, 2012 #4

    CAF123

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    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Yes.
     
  6. Sep 5, 2012 #5
    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Okay thanks for the help, I am feeling kinda dumb now that I know it is that simple.

    Next question:

    A 60kg snowboarder accelerates down a 32 degree slope at 3.0m/s2. Calculate μk.

    Attempt:

    Obviously since there is an acceleration, I know that the forces in the x direction acting on the snowboarder do not equal zero. When setting up a free body diagram I have FN acting upward, Fg acting "downward" (but not on the y axis), then Fx in the positive x direction, and fx in the negative direction.

    I then get the following:

    Ʃy= FN-mg*cos∅=0;
    so FN=(60kg*9.8m/s2)*(cos 32)=498N.

    Ʃx=mg*sin∅-[itex]\mu[/itex]kFN=ma.

    Is this correct so far? Would it then just be a matter of moving everything around algebraically and solving for [itex]\mu[/itex]k?
     
  7. Sep 5, 2012 #6
    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Oops, *centripetal
     
  8. Sep 5, 2012 #7

    CAF123

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    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Yes, simply solve for [itex]\mu_k [/itex]
     
  9. Sep 5, 2012 #8
    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Okay, getting the hang of this.

    Last question:

    A baseball player initially running at 3.4m/s slides to a stop at third base in 1.2 seconds. Calculate the μk between him and the ground.

    Attempt:

    I'm not sure how to approach this problem. 3.4m/s divided by the 1.2s will give me the acceleration I think (or I guess deceleration in this case). Other than that I am at a loss
     
  10. Sep 5, 2012 #9

    CAF123

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    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    The frictional force provides the negative acceleration, necessary for him to come to a stop. Using the negative acceleration calculated above, you can find [itex] \mu_k. [/itex]
     
  11. Sep 5, 2012 #10
    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Well I know that f=[itex]\mu[/itex]k*FN, but I don't have a mass in order to calculate FN, nor do I have f. What equation can I use to solve the problem?
     
  12. Sep 5, 2012 #11

    CAF123

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    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Simply use [itex] F = ma. [/itex] You know the only force acting horizontally on the baseball player as he slides (force of friction). This is your [itex] F.[/itex] Now sub in what [itex] F [/itex] is equal to and what do you notice about [itex] m [/itex]?
     
  13. Sep 5, 2012 #12

    vela

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    Re: Help with physics homework regarding friction, tension, springs and centrifugal f

    Your teacher probably encourages you to work problems out using symbols first and plug numbers in at the end. This is one of the reasons why.
     
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