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Homework Help: Weight of an object sliding on an incline

  1. Apr 3, 2012 #1
    First I would like to appologize for my english, i'm from belgium and I don't often use it to speak about physics.
    Second, I don't know if i'm in the good thread to speak about that.

    Ok, I need a little help,

    I have a problem where an object slidind on a incline plan.

    I would like to know its weight but i 've found 2 formules(?)

    P= m.g.sin θ
    P= m.g.cos θ

    Could you explain me how use them in the right way ?

  2. jcsd
  3. Apr 3, 2012 #2
    Take a look at this ramp I drew. It shows how you can use the angle that the ramp makes with the flat floor to break the force of gravity into two components: the force normal to the ramp's angled surface and the force along the angle of the ramp.
    [tex]F_x = mgsin(\theta)[/tex]
    [tex]F_y = mgcos(\theta)[/tex]
    Notice that the "x" component (along the angled ramp) uses a sin instead of cos. And the opposite is true for the "y" component (normal to the angled surface). These two equations are found through the definition of a cosine and sine of an angle, using the triangle I drew.

    Attached Files:

    • ramp.JPG
      File size:
      16.6 KB
  4. Apr 3, 2012 #3
    Think of simpler example first.
    If you're in an elevator on a scale and the elevator is accelerating downward let say x m/s^2, what is your weight as shown by the scale .

    In inclined plane too, find what is the downward acceleration.
  5. Apr 3, 2012 #4
    Thank you very much:-)
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