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Setting up systems of forces and free body diagrams

  1. Sep 20, 2015 #1
    1. The problem statement, all variables and given/known data
    Suppose I've got a static mass on a downward plane. (For instance a car on a drive way). How would you set up this free body diagram?

    2. Relevant equations
    F = f1+f2....etc

    3. The attempt at a solution
    image.jpeg
    My preference is A, but I've seen it displayed in examples in my book as B. Is there and difference?
    In the instance of this being a mass, I used A and said W could be broken down into components of W (for Wx and Wy) and that the normal force could be broken into X and Y as well for normal and frictional forces. The problem I see is that my X components will be opposite of how they are in real life. (I.e. The weight ought to be pointing toward the left because it will roll down the plane and friction ought to be pushing it up the plane). However, if you translate the system from a normal scenario to an inclined scenario (i.e. The left sketch to A) then it ought look like A.
     

    Attached Files:

  2. jcsd
  3. Sep 20, 2015 #2
    No, image B is correct. The normal force is perpendicular to the incline, but the weight always points directly downwards (towards the center of the earth). As for your comment regarding the direction of the weight being in the direction of motion: if you project the weight vector onto the incline, you notice that some component of the weight is going to push the car to the left in image B. That isn't the case in image A.
     
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