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Newton's second law of motion questions

  1. Feb 6, 2012 #1
    physics help please......

    a person with a mas of 75.0 kg is wearing roller blades and stands at rest on top of a long hill. the hill makes an angle of 15.0 above horizontal. assume the coefficient of friction to be .10.

    a. determine the acceleration of a the person as they roll down the hill.. for this one i got around 1.55 m/s^2.. but my other friend got a different answer soo i am wandering if anyone could help me..

    b. assuming the hill to be 300. meters long, that the person starts from rest, and that the acceleration remains constant the entire length of the hill, determine the person's speed at the bottom of the hill.

    for b .. are we suppose to solve for distance or just use 300 in a kinematic equation?
     
  2. jcsd
  3. Feb 6, 2012 #2

    Doc Al

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    Staff: Mentor

    Show how you got your result.

    You're given the distance, so use it.
     
  4. Feb 6, 2012 #3
    for a. i set up like this :
    Fnet= Fll - Friction
    ma=sin(15)*(mg)-u*(mg)
    (75kg) a=sin(15)(75kg)(9.81)-(0.10)(75kg)(9.81)
    a= 1.55 m/s^2

    is it right..

    and to do B i have to know acceleration from a
     
  5. Feb 6, 2012 #4

    Doc Al

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    Almost right. The friction force equals μN, where N is the normal force. Since it's on an incline, the normal force doesn't just equal mg. What does it equal?
     
  6. Feb 6, 2012 #5
    it equals the weight.. which is mass*gravity.. thats wt i did..
     
  7. Feb 6, 2012 #6

    Doc Al

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    That would be true if the surface were horizontal, but it's not. To find the normal force, consider force components perpendicular to the surface. They must add to zero.
     
  8. Feb 6, 2012 #7
    yeah but we dont have to use the perpendicular fore.. we could just use the parallel force...
    and i am really confused now... can u please help me out a little more
     
  9. Feb 6, 2012 #8

    Doc Al

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    You are using the wrong normal force. The normal force depends on the angle of the surface.

    Read this: Inclined Planes
     
  10. Feb 6, 2012 #9
    the normal force is the weight .. and it doesn't depend on the angle.. okay if i am using the wrong normal force wts the correct one then..
     
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