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Homework Help: FBD Clarification

  1. Mar 10, 2005 #1
    Hey, i need some insight for free body diagrams.
    Say your in an elevator and your nomral wieght is 500N and there also is a scale.

    The scale starts to read 450N. Which means it would be accelrating downwards. So for the FBD, 500N would be in the middle. Then would there be an arrow pointing down being Fg as 450N?? I guess you would assume no air resistence, so would there be any value for Fn??

    Also if the scale reads 0, would Fn=Fg??
    thanx again.
  2. jcsd
  3. Mar 11, 2005 #2

    Chi Meson

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    As you might already be aware, whatever the scale reads, that IS the normal force on you.

    If you are not accelerating and the scale reads 500 N, then this is your weight (force due to gravity). The scale reads 500 N because this is the normal force that is currently balancing your weight.

    If the scale reads 450 N, your weight does not change; your weight is still 500 N (down), but now the normal force is only 450 N (up). You are correct, you must be accelerating down because your net force is 50 N down.

    Think again about that last statement. If the scale reads zero, what's pushing you up? What's still pulling you down?
  4. Mar 11, 2005 #3


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    I'm not sure what you mean by "500N would be in the middle"!

    Your weight is the downward force on you. The weight read on the scale is the force of the scale upward on you. They are not the same because you are accelerating- there is a net force.
  5. Mar 11, 2005 #4
    For free body diagrams the mass or weight of the object goes in the middle of any of the forces acting on it so the 500N is ann objcet that ways 51Kg. And then there would be an arrow going up from it being the normal force and one down being gravity. for these questions anyways.

    Ok, and for the scale reading 0, Well, air resitence would be holding you up, and garvity would be forcing you down. So i guess assuming there is no air resitence, it would just be an arrow down being Fg??
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