Mercury in Freefall: The Impact on Elevator Mechanics Explained

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    Elevator Falling
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Discussion Overview

The discussion revolves around the behavior of a drop of mercury in an elevator during freefall, exploring the physical implications of this scenario on the shape and motion of the mercury. Participants examine concepts related to fluid dynamics, surface tension, and the effects of gravity on the mercury's behavior in a non-gravitational environment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the mercury will form a nearly perfect sphere and may float upwards slowly as the elevator falls.
  • Others question the mechanism behind the upward motion of the mercury, suggesting it may oscillate after touching the top of the elevator.
  • One participant suggests that the initial flattening of the mercury blob is due to gravity or the elevator floor, and that it gains a "kick" when gravity is effectively "switched off."
  • Another participant expresses confusion about the concept of the mercury receiving a kick, comparing it to water in a dropped plate, and emphasizes the importance of surface tension in this context.
  • Some participants discuss the implications of the mercury's shape change and the resulting motion, questioning whether this behavior is analogous to that of ball bearings dropped on a plate.
  • There is mention of a restoring force acting on the ball bearings, which could provide a similar kick, though this effect may be negligible depending on the floor material.
  • One participant clarifies that the kick occurs before the mercury separates from the floor, likening the situation to a spring that is compressed by gravity and then allowed to extend in freefall.

Areas of Agreement / Disagreement

Participants express differing views on the mechanics of the mercury's behavior in freefall, particularly regarding the concept of the "kick" and its implications. There is no consensus on the exact nature of the forces at play or the resulting motion of the mercury.

Contextual Notes

Participants highlight the assumptions regarding the non-wetting behavior of the mercury and the conditions under which the comparisons to water and ball bearings are made. The discussion remains open to further exploration and clarification of these concepts.

hav0c
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There is a drop of mercury in an elevator (flat on the ground).
during freefall what will happen to the mercury.
 
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It should form a (nearly) perfect sphere, probably floating upwards very slowly.
 
mfb said:
It should form a (nearly) perfect sphere, probably floating upwards very slowly.

what causes it to float upwards
plus if it floats upwards then it should oscillate inside the elevator after touching the top.
 
I think the idea is that as the elevator starts to fall, the formerly-flat blob of mercury will fatten out to form a spherical shape. In the process of doing this it will effectively "push off" from the floor.
 
Initially, the drop is flattened a bit by gravity (or by the floor pushing it, if you like). If you "switch gravity off", the mercury gets a small kick from this force, until it loses contact with the floor. After some initial, damped oscillation, it reaches a stable spherical shape.

plus if it floats upwards then it should oscillate inside the elevator after touching the top.
I would not expect that it reaches the top before the elevator crashes, but this might require some simulation (or experiment ;)) to find out.
 
I don't understand what is mean by switching off gravity gives the mercury a kick.
If I simply drop a plate with water on it I don't think the water gets a kick...it just falls like the plate.
 
Emilyjoint said:
I don't understand what is mean by switching off gravity gives the mercury a kick.
If I simply drop a plate with water on it I don't think the water gets a kick...it just falls like the plate.

The assumption is that the mercury is not wetting the floor of the elevator. It is beading up. If you want to compare with a plate of water, you will need to grease the plate or otherwise arrange for it not to be wetted. That way the water will "bead up".

Now before you drop the plate, a water bead will be flattened somewhat into a pancake shape, right?

And after you drop the plate that bead will draw itself together into a sphere, right?

Where is the center of gravity of the pancake shape with respect to the plate?

Where is the center of gravity of the spherical shape with respect to the plate?

Does this imply that the water bead has moved away from the plate?

Does this imply that the water has acquired a velocity relative to the plate?

Where did that velocity come from?
 
So this explanation must be due to surface tension?
If I drop a plate with ball bearings on it the ball bearings don't get a kick, do they?
Where is the centre of mass of the plate with respect to the drop...
 
If I drop a plate with ball bearings on it the ball bearings don't get a kick, do they?
They will compress the floor (and in theory even themself) a bit, the corresponding restoring force would give them a small kick, too. But the effect would be smaller, and negligible if the floor is some hard material.
 
  • #10
mfb said:
They will compress the floor (and in theory even themself) a bit, the corresponding restoring force would give them a small kick, too. But the effect would be smaller, and negligible if the floor is some hard material.

I'm guessing this kick is the point were the mercury separates? Also off on this no-gravity kick thing.
 
  • #11
The kick is done before separation. It does not come from gravity, but the situation before the elevator falls is necessary to get it. Think of it like a spring which gets compressed by gravity, when the elevator is resting. In free fall, the compressing force is gone, and the spring extends.
 
  • #12
mfb said:
The kick is done before separation. It does not come from gravity, but the situation before the elevator falls is necessary to get it. Think of it like a spring which gets compressed by gravity, when the elevator is resting. In free fall, the compressing force is gone, and the spring extends.

Easier to visualize than to describe for me, but I understand what you're saying.
 
  • #13
mfb said:
The kick is done before separation...
now its all clear... http://kontaktniy.org
 
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