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Elevator Problems (Help please)

  1. Nov 14, 2006 #1
    I need a little help with this problem. I know what the answer is, but not how to get it.

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    A passenger in an elevator has a mass of 100 kg. Calculate the force in newtons exerted on the passenger by the elevator if the elevator is moving upward with an acceleration of 30 cm/s^2.
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    Can anyone help me please?
     
  2. jcsd
  3. Nov 14, 2006 #2

    Hootenanny

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    I would first recommend drawing an FBD. If the elevator is accelerating at 30cm/s2, what is the acceleration of the person?
     
  4. Nov 14, 2006 #3
    The person should have an acceleration of 30 too, since he's inside the elevator. This gives me:

    Sum of Forces = mass x acceleration
    Sum of Forces = 100 (30)
    Sum of Forces = 3000

    When you subtract the force from gravity (-980) the normal force acting on the guy from the elevator is 3980 N, which is much too high. (the correct answer is 1010 N)

    I could be misunderstanding this, I'm not great at physics.
     
  5. Nov 14, 2006 #4

    Hootenanny

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    Okay, your on the right lines but not quite there, did you draw a FBD? Let's first look at the forces involved, let the acceleration force be Fa and let vertically upwards be defined as positive; we therefore have;

    [tex]\sum\vec{F} = F_{a} - mg[/tex]

    Does that make sense?
     
  6. Nov 14, 2006 #5
    No, I'm sorry, it doesn't. The only formula i've learned so far is "force equals mass times acceleration" Can you show me how you got the other formula?
     
  7. Nov 14, 2006 #6

    Hootenanny

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    There is no formula required for this, the above step was just summing the forces or finding the net force.
     
  8. Nov 14, 2006 #7
    Okay, so mg = 100(-9.8) = -980. I'm not sure about the force of acceleration. Is is 30?
     
  9. Nov 14, 2006 #8

    Hootenanny

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    30 cm/s2, you must first convert this to m/s2. So putting our forumale together;

    [tex]\sum\vec{F} = m\vec{a}[/tex]

    [tex]F_{a} - mg = ma[/tex]

    Do you follow?
     
  10. Nov 14, 2006 #9
    so:
    Fa - mg = ma
    Fa = ma + mg
    Fa = (100)(3) + (100)(-9.8)
    Fa = 680

    That doesn't seem right.
     
  11. Nov 14, 2006 #10
    Ah. I got it. The problem was the cm - m conversion. Thanks for the help though. My teacher's a psychopath.
     
  12. Nov 14, 2006 #11

    Hootenanny

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    No problem. Why do you say your teacher's a psychopath?
     
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