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Homework Help: Elevator acceleration help

  1. Feb 15, 2010 #1
    A 70kg person is in an elevator the elevator has an acceleration of 3m/s/s DOWNWARD. What force will the floor exert on the person.

    I am confused on the the equation. So far I think (mass)(acceleration)=(mass)(gravity)-(normal force). I know the mass is 70kg or 700N. I also know the acceleration is 3m/s/s. so i started to fill in my equation an i got -210=700-(Normal Force). But after solving I don't see that method resulting in the correct answer. HELP
     
  2. jcsd
  3. Feb 15, 2010 #2
    Re: Elevator

    You put a minus sign in front of the acceleration, so your positive direction is up and the
    negative direction is down. Gravity is also downwards, so it should get a minus sign as well.
    The normal force is upwards, so it should not get a minus sign.
     
  4. Feb 15, 2010 #3
    Re: Elevator

    i am still confused, So i worked it out the way you told me, but since i am looking for the amount of force the floor is exerting on the person should i find the normal force. or should it just be Mass x Gravity

    Here is my work
    (Mass)(Acceleration)=Normal Force-(Mass)(Gravity)
    -210=Normal Force -700
    Normal Force= 490
     
  5. Feb 15, 2010 #4
    Re: Elevator

    The amount of force the floor is exerting IS the normal force, so your answer is
    correct.
     
  6. Feb 15, 2010 #5
    Re: Elevator

    thanks for the help. really appreciate it
     
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