Scale Reading of Normal Force: Due by Midnight!

  1. Remember that a scale records the value of
    the normal force, not a person’s actual weight.
    Draw a FBD. Rotate your coordinate system.

    A 61 kg student weighs himself by standing
    on a scale mounted on a skateboard that is
    rolling down an incline, as shown. Assume
    there is no friction so that the force exerted
    by the incline on the skateboard is normal to
    the incline.
    The acceleration of gravity is 9.81 m/s

    What is the reading on the scale if the angle
    of the slope is 29◦?
    Answer in units of N

    I have my free body diagram with Fn going up and to the left and Fg gong straight down. I do not understand how to rotate the FBD or what that would accomplish...
  2. jcsd
  3. PhanthomJay

    PhanthomJay 6,187
    Science Advisor
    Homework Helper
    Gold Member

    Your FBD is correct. Now break up the weight force into components parallel and perpendicular to the plane, and cross out the initial weight vector. The sum of forces perpendicular to the incline must equal zero, since the skateboarder does not lift off the incline in that direction.
  4. much thanks! mission accomplished
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