What Happens to a Candle and Mercury in a Freefalling Elevator?

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Discussion Overview

The discussion revolves around the behavior of a lit candle and a drop of mercury in a free-falling elevator. It explores the implications of free fall on combustion and fluid dynamics, considering both theoretical and practical aspects of the scenario.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the blob of mercury would become more spherical due to reduced gravitational effects, while noting that air resistance still plays a role in the elevator's descent.
  • Another participant posits that the candle would struggle to burn effectively in free fall, as the lack of rising hot gases would hinder combustion, although they suspect it might still burn to some extent.
  • A different viewpoint argues that since both the air and the candle are in free fall, there would be no change in the behavior of the candle, implying that the hot vapors would not rise and could extinguish the flame.
  • One participant refines this idea by suggesting that while the elevator is not in perfect free fall, the candle might remain lit if the elevator is hermetically sealed.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of the candle in free fall, with some believing it would extinguish due to the lack of rising hot gases, while others argue it may still burn under certain conditions. The discussion remains unresolved regarding the exact outcomes for both the candle and the mercury.

Contextual Notes

Assumptions about the elevator's conditions, such as whether it is hermetically sealed or the effects of air resistance, are not fully explored, leading to uncertainty in the conclusions drawn by participants.

Julian Solos
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"There is a lit candle in an elevator mounted on a bracket attached to the middle of one wall (say, 2" from the wall). A drop of mercury
is on the floor. The cable snaps and the elevator falls.
What happens to the candle and the mercury?"

From: http://research.microsoft.com/~mbj/Smiley/Joke_Thread.html


Maybe the question was meant to be a joke, but it doesn't have to be.
 
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The blob of mercury becomes more spherical, as surface tension no longer contends with much gravity to shape it. Since the elevator descends with acceleration due to g-air resistance, and the mercury has negligible air resistance, there is still a little pressure deforming it.

I assume the candle is lit, otherwise the question is dull. Candles depend on hot gases rising to feed the flame. In this case, the candle is nearly experiencing freef-fall, so there is little gravity to cause the hot vapor to rise in relation to the cool air. I suspect it would still burn, but not nearly as well. Just a little preferential directionality can cause a strong fluidic flow. If you've ever done the experiment with a candle burning at the bottom of a long glass tube, you know what I mean.

Njorl
 
but the air would be falling at the same rate as the cadle, so i think the cadle would have no change.
 
Originally posted by Gara
but the air would be falling at the same rate as the cadle, so i think the cadle would have no change.

The air and the candle would both be in freefall. There would be no reason for hot vapors to rise. If the vapors don't rise, the candle goes out. As noted though, the elevator won't quite be in freefall, so the candle might stay lit. This is assuming a hermetically sealed elevator though.

Njorl
 

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