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wlayton27

5 Fun Physics Phenomena

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Veritasium just recently posted a new video titled "5 fun physics phenomena:"

 

 

He posted 5 little physics "party tricks" and asked the viewers for explanations for what causes those quirky results to occur. He plans on posting his answers next week, but in the meantime I thought this might make a fun conversation.

 

#1: The balancing cane: I believe this is caused by simply the bodies reaction to the weight of the cane at the two fingertips as the hands move inwards while holding the cane. If the cane feels heavier in one hand, the person will automatically move the opposite hand inwards at a greater rate to compensate and maintain balance.

 

#2: The phone flip: I like this one and have noticed it quite a bit myself (I like flipping things). Flipping the phone along the flat face (directional axis -- "spinning") is well balanced because the mass is localized near the fulcrum along the axis (a radial rotation), and flipping the phone along the top face (lateral axis -- "rolling") is also well balanced but this time it's because the mass is localized closest to the fulcrum along the radius (an axial rotation, like an American football spiraling in the air). Flipping the phone along the side face (longitudinal axis -- "tumbling") is less well balanced than either rolling or spinning, and the phone will naturally be inclined to precess towards either a roll or a spin.

 

#3: The static water faucet: This one reminds me of Lord Kelvin's water dropper. He already explained that the hydrogen bonds of the water cause the water molecule to rotate with the positive side (hydrogen atoms) facing towards the negative electrostatic charge on the plastic cup. While the negative charge causes the oxygen atoms to be repelled, the hydrogen atoms are closer to the cup and feel more attraction. The same happens with a positively charged object (e.g. a ball of fur). The water is attracted to a statically charged furball because the oxygen atoms are then closer and have a force of attraction that is greater than the repulsion of the hydrogen atoms.

 

#4: Magnetic cereal in water: Probably something to do with the iron content of the cereal? It's listed on the nutrition facts on the side of the box. :D

 

#5: Teabag rocket: Seems the same as the Chinese sky lanterns. The hot air inside the bag becomes less dense than the surrounding air and rises, bringing the ashes of the tea bag upwards with it.

 

Let us know what you think the solutions will be.

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I think I agree with most of your answers, but a little more on Problem 2: A pretty standard example of stable/unstable rotation, here's some maths on why this happens

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The explanations are a bit simpler for #1 and #3, I believe.

 

#1 - The lighter end creates less friction against the finger on that side, opposite for the heavier end. This trick will only work if the entire surface of the cane has a uniform friction coefficient.

 

#2 - You got it. Flipping the phone that way has the weight distributed unevenly *along* the axis of rotation, not just around it.

 

#3 - The cup has a large charge, the water (having just exited an earthed plumbing system) has no or little charge. Attraction, nothing to do with the "alignment" of the molecules.

 

#4 - Essentially, yes. It's an experiment we did in high school (not the part about trying to remove iron from food, just the part about detecting it.) http://bit.ly/1of3dcp

 

#5 - Nailed it. The effect is amplified because once the teabag paper turns to ash, it's even lighter again.

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#1: cane:

i would guess its a combination of friction AND leverage. there is greater downward force and hence friction at the finger closest to the CoM, whilst the other finger experiences less resistance due to a fulcrum being created at the opposing finger.

 

consequentially, some of the energy used by the finger furthest from the CoM to overcome friction is transferred into downward force at the other finger, to a degree that is always proportional to the differential of the fingers' distances from the CoM, so therefore, the system stays in equilibrium -- irrespective of speed.

 

#2: phone:

anyone here ever sat on a chair and spun whilst holding their arms and legs outstretched -- and then experienced rapid acceleration upon pulling their limbs into the centre?

 

i think this happens,

 

firstly, because greater leverage along the length of the phone makes it FAR more difficult to confine all of the intended initial momentum to the correct axis. (conversely, in the case of flipping, the initial force imparted is somewhat stabilised in the correct direction much in the same way a tightrope walker holds a plank of wood)

 

secondly, as soon as the tiniest tiniest 'corkscrew effect' is produced, it will never be reversed, because it is far 'easier' for the mass contained within the corners of the phone to be accelerated about a smaller radius of rotation.

 

also i wonder if air resistance eventually becomes a factor? when flipping, there is comparatively more air to push around to achieve the same rotational speed as rolling or spinning. also theres more chance that small chaotic pockets of air pressure will not cancel each other out as well along the shorter left to right plane, encouraging spinning in that axis, which would contribute to the corkscrew effect.

 

which makes me wonder: would flipping be marginally easier in space? or in a vacuum on earth?

Edited by @~thehung

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1: cane : saw that same thing on QI a while back. Mr Fry thought he was pretty clever with it and for the life of me

I can't remember what the reasoning he gave was :P

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Answers are up already:

 

 

I was a bit surprised by the solution to the static water faucet, and also by the added effect of diamagnetism of water that applied to the floating cereal. He also had a much shorter explanation of the phone flip than I have ever heard. :D

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