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Homework Help: Trying to solve an elevator problem

  1. Feb 6, 2013 #1
    1. The problem statement, all variables and given/known data
    An elevator accelerates downward at 3.2 m/s2. What force does the floor of the elevator exert on a 46 kg passenger?

    2. Relevant equations

    T being tension

    3. The attempt at a solution
    I don't know if the tension of the elevator line is 9.81N, and in the passenger I got 147.2 N for his force(using the acceleration of the elevator.
  2. jcsd
  3. Feb 6, 2013 #2
    You don't need the tension of the elevator cable here since it doesn't directly act on the passenger.

    Draw a free-body diagram for the passenger and use Newton 2.
  4. Feb 6, 2013 #3
    so i basically get the force of the person by, multiplying 49kg*3.2m/s^2 right? if so, then how do I get the force of the floor? it is less because the elevator is going down right?
  5. Feb 6, 2013 #4
    What are the 2 forces acting on the passenger?
    Draw these on a diagram and add them.
    Their sum will be the quantity you calculated.
    ( Make sure that your signs are correct here ).

    You are right in saying that the normal force will get less.
  6. Feb 6, 2013 #5
    so, the answer would be 137.39N ? because of the substraction of the normal force of the man and the default force 9.81N?
  7. Feb 6, 2013 #6

    The normal force on the passenger acts upwards, and the weight acts downwards.
    The sum of these is the quantity you calculated in #3, ( from F(net) = ma ).

    You can work out the normal force from this.
    Choose one direction as positive ( eg the downward direction ) and make sure your signs are correct.
  8. Feb 6, 2013 #7
    so it would be 304.06 - 147.2 = 156.86N
  9. Feb 6, 2013 #8
    Where does the 304.06 come from?

    You should have something like
    mg - n = ma
  10. Feb 6, 2013 #9
    well, is (9.81-3.2)*46= 304.06 and that's the final answer.
  11. Feb 6, 2013 #10
    Yes :)
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