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Homework Help: Newtons third law, elevator question (free body diagram)

  1. Apr 16, 2012 #1
    1. The problem statement, all variables and given/known data

    An elevator containing 3 passengers(with a mass of 72kg 84 kg and 35 kg) has a mass combined of 1030 kg . The cable attached to the elevator exherts an upward force of 12000 N. But the friction opposing the motion of the elevator is 1400 N.

    a) Draw a free body diagram for all the forces acting on the elevator.

    2. Relevant equations

    3. The attempt at a solution

    Now since net force is greater for upward motion, we can assume the elevator is going up, since gravity + friction is still less than the upward force exherted, so elevator is going up,
    but my question is...in the free body diagram..do i put both friction and force of gravity as a downward force???

    Or should i just remove force of gravity and just put force of friction in the freebody diagram as a downward force?

    The free body diagram i have in mind is in attachment, and thats what it looks like now....as always you dont need to download attachment to view it.


    Attached Files:

  2. jcsd
  3. Apr 16, 2012 #2


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    Homework Helper

    I believe you should be drawing both "gravity" and friction - down - but only Applied force up.

    The net force is the vector sum of the three real forces - but I am not sure it is generally shown on these diagrams.

    Also - you can do MUCH better with the arrows. The applied Force vector should be nearly 9 times as long as the friction Force.
    How does the weight force compare in size?
  4. Apr 16, 2012 #3
    well this is not the actual diagram, its not to scale, the actual diagram is done in paper, i just sort of did a quick sloppy version to post here
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