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B Elevator free fall

  1. Aug 10, 2017 #1
    Hi guys,

    I know this is a really simple question, but I'm just making sure I have this concept down:

    Scenario 1. An elevator accelerates down at 9.8m/s^2.
    Scenario 2. The elevator cable is cut.

    If someone is inside an elevator, scenarios 1 and 2 would feel exactly the same to them, correct?


  2. jcsd
  3. Aug 10, 2017 #2


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    Assuming no other forces on the elevator aside from the cables and the Earth's gravity then you are correct.

    If the elevator accelerates downward at 9.8 m/s^2 then it must be getting no support from its cables. Whether this is because the cables go slack or because they have been cut is pretty much irrelevant.
  4. Aug 10, 2017 #3
    Is the 9.8m/s2 plus gravity, because if the cable is cut it gets 9.8m/s2 acceleration, anyway... because if you accelerate an elevator at 9.8m/s2 in space would be the same as cutting the cable on Earth.
  5. Aug 10, 2017 #4


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    No. Cutting the cable on earth puts the elevator in free fall: objects inside the elevator will float weightless. Accelerating at 1g in space is the same as the elevator sitting still on the surface of the earth.
  6. Aug 10, 2017 #5

    (normal force) - (force accelerating elevator down) - (gravity) = - (gravity)
  7. Aug 10, 2017 #6


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    What normal force?
    What force accelerating the elevator down, other than gravity?
    And why have -gravity on both sides of the equality?
  8. Aug 10, 2017 #7
    The "force accelerating elevator down" is what is moving the elevator down at an acceleration of 9.8 m/s^2, while the cable is still attached.

    I have gravity on both sides since I was comparison Scenario 1 and Scenario 2.

    Am I understanding this concept completely wrong?
  9. Aug 10, 2017 #8


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    Gravity alone would make the elevator accelerate downward at 9.8 m/s^2. Are you saying that there is a second force in addition to gravity that is pulling the elevator downward at a total of 19.6 m/s^2?
    It is not clear what question you are trying to ask yet.
  10. Aug 10, 2017 #9
    Show us your free body diagram for this scenario.
  11. Aug 10, 2017 #10
    Okay, let me reiterate this whole thing:

    So let's say an elevator is hanging by a cable. It is not accelerating. Although there is gravity, the cable supports the weight of the entire elevator(this part I know I'm right for sure). Now let's say this elevator was lowered at an acceleration of 9.8 m/s^2. Since this is the same acceleration as gravity(free fall), would this have the same effect as just cutting the cable of the elevator?
  12. Aug 11, 2017 #11


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    As I said in #2, a slack cable (required to achieve this downward acceleration) and a cut cable have the same effect.

    There is other "downward force" involved. And no unidentified "normal force" involved. However, if you want to toss in additional forces on the elevator that all sum to zero, that works too.
    Last edited: Aug 11, 2017
  13. Aug 11, 2017 #12


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    Under free fall conditions, the elevator AND the person will be accelerating at the same rate. IF you just 'accelerate the elevator' out in space then the passenger would be left behind and end up against the ceiling of the box - just as if the elevator were suspended (upside down) on Earth. The passenger would not feel weightless.
  14. Aug 12, 2017 #13
    Okay, thanks everyone!
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