Candles in an Elevator
If you are holding a candle in an
elevator on the 11th floor(a lit candle) & the cable
breaks, does the candle remain lit as you plummet to the
bottom? Why or Why not?
Thanks for your question.
My answer would be that since the elevator is a semi-sealed system,
we assume there would be no major drafts as you plummet to your doom.
Assuming no major drafts, the candle would continue burning (at least
until the paramedics arrive).
Seriously, you can ask you folks about this. If they drive an automobile
and stick a match to light a cigarette, even though the semi-sealed
system (car) is moving (here horizontally instead of vertically) the
match would remain lit until it either runs out of fuel (end of match
stick, or runs out of oxygen). The other way the flame could be extinguished is
to open a window and allow the semi-sealed system to be opened.
Naturally a strong draft would extinguish the flame, as the system is
not sealed and the lit match experiences actual movement past a large
volume of air (something we call wind). You can mimic the wind by
blowing a large volume of air past the match also.
A question for you: when you 'blow out' a candle, what exactly are you
doing? How is the flame extinguished? How about when you extinguish a
flame with water? Describe why the flame is no longer burning. Please
send you replies here. Thanks for using Newton!
Well, I have heard about doing this experiment with a candle and a
large (draft free) enclosed container. The result is supposed to be that the
candle goes out almost immediately and the rational I have heard makes
sense. The explanation is goes back to Galileo's discovery that similar objects
of different masses fall at the same rate. In your enclosed elevator t
there are three objects of importance and they all fall at the same rate.
They are: the candle (and flame); the "normal" or unheated/unburned air;
and the heated or burned air (oxygen depleted). Normally the heated
air would rise, since it is _LIGHTER_ (or less dense) than the cool
air. As it rises it is replaced with fresh air with a supply of oxygen
to sustain the flame. In the free-falling, closed container everything
falls at the same rate -- i.e., there is NO RELATIVE motion of the
flame, the cool air and the oxygen depleted hot air. The result is that
the oxygen in the vicinity of the flame is rapidly depleted and the
flame dies due to oxygen starvation.
Now that we have two opposing views it is time for peer review.
gregory r bradburn
I believe your assertion that similar objects of different masses
fall at the same rates is true in a vacuum, but clearly a candle
could not burn in a vacuum.
Wow! I am with Dr. Blackburn's beautiful reply. Fires clearly
depend on the flow of combustion reactants into and products out of the reaction
zone. This flow also clearly depends normally on the rising of air as it
is heated (try lighting a fire in the fireplace with the chimney damper
closed). But hot air only rises relative to cold air when it *weighs* less,
and in the elevator in free fall everything is weightless. Of the varying
terminal velocities of candle, air, and elevator in atmosphere only the
last is relevant, since all objects are enclosed in the elevator. When the
terminal velocity of the elevator (at a guess several hundred miles an
hour) is reached weight will return and the candle can burn efficiently
again. You could try the experiment in the Space Shuttle, but I would not
think they would allow open flames.
All objects will fall at the same rate whenever the net forces
acting on them are the same. When not in a vacuum, i.e., falling
through the air, there are viscous effects which affect the rates.
However, this problem specifies a closed container so the viscous
effects act only on the container. I assumed that the mass of
the container is large enough and its shape is such that its
terminal velocity is large -- actually I had not considered
terminal velocity at all until Chris mentioned it.
Of course I had also not considered trying to light a candle
on the space shuttle -- the problems you run into with space travel!
gregory r bradburn
I will have to cast my vote with Chris Grayce. See my simple
response to #130 in the Physics Archive Section.
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Update: June 2012