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Re: [Phys-l] ball floating in elevator



An interesting variation on you're a>g demonstration is to simply take a paper cup filled with water and let it drop to the floor. Students are surprised that no water leaves the cup until the cup hits the floor (or wastebasket).

We all do the demonstration where we whirl a pail of water in a vertical circle. If the students are asked what will happen if the bucket is whirled such that v^2/r < g, they invariably say the water will spill out of the bucket. In actuality it doesn't - the bucket falls at the same rate as the water - but now in a parabolic arc instead of a circle.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Ken Caviness
Sent: Tuesday, April 21, 2009 12:43 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] ball floating in elevator

As Bob says, just think of the acceleration of the elevator as adding to g, the "acceleration of gravity".

Since water falls and air-filled balls in water rise under the influence of gravity, they will do the same in the accelerating elevator, although we should use (g+a) instead of g everywhere. For a floating object we have equilibrium with Weight (downward) + Buoyancy_Force (upward) == 0, so in magnitude, mg == rho g V, and the actual value of g cancels out, so it makes no difference whether the experiment is done on terra firma, or in an upward accelerating elevator (replacing g by (g+a)), or on the moon, etc.

Same thing if the elevator is accelerating downwards: replace g by (g-a). Nothing unusual to film as long as the object is floating, which will remain true if a<g.

But if a>g, if the elevator is accelerating downwards at rate that exceeds g, the acceleration experienced during freefall, the "effective gravity" is _upwards_ instead of downwards! The forces on the ball are no longer in equilibrium, the water and ball will accelerate towards the ceiling of the elevator, SPLASH! Something to film after all. ;-) Or one could quickly turn the apparatus upside down and observe the ball floating on the bottom of the water which held upside down in the bucket! Film it quickly, before the elevator crashes.

Finally, it would be much more interesting if water and ball were sealed in a tall glass tube from floor to ceiling in the elevator. As long as the elevator's downward acceleration is less than g, the ball stays at the top of the tube. If the elevator's downward acceleration exceeds g, the ball "rises" to the bottom of the tube of water (the water & tube are "falling" toward the ceiling).

Ken

caviness@southern.edu

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of LaMontagne, Bob
Sent: Tuesday, April 21, 2009 11:26 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] ball floating in elevator

Obviously the ball goes up with the elevator.

Bob at PC
( mg = rho g V, g increases on both sides of the equation, so V
doesn't change?)

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Anthony Lapinski
Sent: Tuesday, April 21, 2009 8:38 AM
To: phys-l@carnot.physics.buffalo.edu; tap-l@lists.ncsu.edu
Subject: [Phys-l] ball floating in elevator

A ball is floating in a beaker of water in an elevator. What happens to
the ball when the elevator accelerates upward? Good question for your
"bright" students!

Think about this before checking out the video below.

http://www.youtube.com/watch?v=OXuYWCWIaJI

I guess you could demonstrate an elevator accelerating downward -- free
fall -- by dropping a (plastic) beaker above a garbage pail to see what
the ball does while falling. Might be hard to observe this fast motion,
so
a video camera would be useful.

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Forum for Physics Educators
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l