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Physics puzzles
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Author:  SpongeBob [ Mon Nov 22, 2010 3:08 pm ]
Post subject:  Physics puzzles
I like puzzles, but the math puzzles in the other thread are too hard for me. So why not start a thread for puzzles in an even more interesting field of study :cool: : physics!

Let's start with this one:

You are in the basement of a building. There are three switches, which are all in the off position. You know that exactly one of the switches is connected to a light bulb in a room in the first floor. However, to check if the light went on, you will need to take the elevator to the first floor and check the room. Now you are allowed to use the elevator only once. How do you figure out which one of the three switches controls the light?
Author:  wms [ Mon Nov 22, 2010 3:37 pm ]
Post subject:  Re: Physics puzzles
A. flip switch 1 to "on"
B. wait several years
C. flip switch 1 to "off", switch 2 to "on"
D. Ride elevator.
* Burned out bulb = switch 1 controls the light
* Bulb on = switch 2 controls the light
* Bulb off = switch 3 controls the light

Is that it? I'm guessing there is a more elegant answer, but I think my solution will work. :)
Author:  Li Kao [ Mon Nov 22, 2010 3:43 pm ]
Post subject:  Re: Physics puzzles
You're close, but a bit destructive. You can simply wait a few minutes until the lightbulb gets warm and check its temperature instead of waiting until it dies.
Author:  Koroviev [ Mon Nov 22, 2010 3:45 pm ]
Post subject:  Re: Physics puzzles
:scratch:
Switch 1 - on for two minutes, then...
Switch 1 - off.
Switch 2 - on
Switch 3 off.

Jump in the elevator (quick!) and go up to the room.

If the bulb is off AND warm - switch 1 controls it.
If the bulb is off AND cold - switch 3 controls it.
If the bulb is on AND not warm - switch 2 controls it.

Is that right?

EDIT: Bummer, beaten to it. :grumpy:
Author:  wms [ Mon Nov 22, 2010 4:36 pm ]
Post subject:  Re: Physics puzzles
Li Kao wrote:
You're close, but a bit destructive. You can simply wait a few minutes until the lightbulb gets warm and check its temperature instead of waiting until it dies.
Ha! I was closer than I thought.

OK, here's another physics puzzle (well, maybe not physics, but some branch of science is involved):

There is a lake. It's like a giant bathtub - no water in, no water out. A guy is in a boat in the lake, with a rock in the boat. The guy takes the rock out of the boat and drops it into the lake; it sinks to the bottom. After the rock is dropped in, is the water level of the lake higher, lower, or the same?
Author:  cyclops [ Mon Nov 22, 2010 5:08 pm ]
Post subject:  Re: Physics puzzles
Suppose for an eay argument that the floor of the pool is horizontal and the sides are vertical. With the rock on the floor the pressure exerted by the area under the stone is bigger than the upward pressure exerted by other parts of the floor. The total upward force exerted by the floor of the pools remains the same. ( If you dont believe try lying under it ). Thus the upward pressure exerted by other parts of the floor has decreased after the rock landed. So the hydrotic pressure and hence the depth has and hence the water level has decreased. Another question is when the water level decreased. When the rock is held under water, when it was released or when it landed.
Author:  jts [ Mon Nov 22, 2010 5:16 pm ]
Post subject:  Re: Physics puzzles
Quote:
After the rock is dropped in, is the water level of the lake higher, lower, or the same?


My answer is...

Lower. The boat displaces the volume of water which is equal to its weight; the rock displaces a volume equal to its own volume; but rocks are denser than water, so the rock displaces more water on the boat than in the water.

On preview - cyclops is wrong :)
Author:  wms [ Mon Nov 22, 2010 5:20 pm ]
Post subject:  Re: Physics puzzles
jts wrote:
Quote:
After the rock is dropped in, is the water level of the lake higher, lower, or the same?


My answer is...

Lower. The boat displaces the volume of water which is equal to its weight; the rock displaces a volume equal to its own volume; but rocks are denser than water, so the rock displaces more water on the boat than in the water.

On preview - cyclops is wrong :)

You are right. Cyclops, you are right too, although your reasoning is much more convoluted. :)

Also, some rocks (such as volcanic rocks with lots of air bubbles) actually are less dense than water, so it's important when giving the puzzle that you mention the rock sinking to the bottom. *ANYTHING* that sinks to the bottom must be denser than water, and thus will make the water level go lower when it is dropped out of the boat.
Author:  DrStraw [ Mon Nov 22, 2010 7:49 pm ]
Post subject:  Re: Physics puzzles
Li Kao wrote:
You're close, but a bit destructive. You can simply wait a few minutes until the lightbulb gets warm and check its temperature instead of waiting until it dies.


What if they are fluorescent bulbs?
Author:  entropi [ Tue Nov 23, 2010 12:43 am ]
Post subject:  Re: Physics puzzles
Another one: you forget the refrigarator open. (Why) Will the room temperature increase or decrease?
Author:  Sverre [ Tue Nov 23, 2010 1:49 am ]
Post subject:  Re: Physics puzzles
entropi wrote:
Another one: you forget the refrigarator open. (Why) Will the room temperature increase or decrease?


The room might become a little cooler in the short term, especially if you unplug the refrigerator, but in the long term the refrigerator will heat the room.

The refrigerator tries to cool point A by dumping even more heat at point B. Opening the refrigerator door will make the refrigerator move heat from A to A, adding a bit of energy in the process. This will heat the room.
Author:  cyclops [ Tue Nov 23, 2010 5:16 am ]
Post subject:  Re: Physics puzzles
Question for entropi. How much of you is on a [font=]green[/font] photon.
Author:  entropi [ Tue Nov 23, 2010 5:37 am ]
Post subject:  Re: Physics puzzles
cyclops wrote:
Question for entropi. How much of you is on a [font=]green[/font] photon.


Photon entropy is independent of color :)

But in fact my nickname refers to entropy in the sense of information theory which is the most basic and my favourite concept for understanding the nature of information, by Shannon.

Sverre's answer to my question seems to be correct.
Author:  cyclops [ Tue Nov 23, 2010 6:36 am ]
Post subject:  Re: Physics puzzles
entropi wrote:
Photon entropy is independent of color :)


Is it? Then it might be k, the Bolzmann constant, 1.380 6504(24) × 10−23 J/K.
Author:  SpongeBob [ Tue Nov 23, 2010 4:11 pm ]
Post subject:  Re: Physics puzzles
cyclops, I liked your reasoning with the pressure, nice.

Well, it seems my first puzzle was not difficult enough for a smart croud like the one here on L19. Watch out: this one is REALLY cool, but also hard to figure out:

You have a ramp and you have two identical cans of soda. If you let both cans roll down the ramp, they will arive at the bottom of the ramp at the same time. Now if you shake one of the cans directly before the experiment, one of them will be faster than the other. Which one is it and why?
And if you do not believe that: try it out!
Author:  Dusk Eagle [ Tue Nov 23, 2010 11:58 pm ]
Post subject:  Re: Physics puzzles
Well, I'm not a physics major, but I just attempted it and
the shaken up one was slower.

I really don't have a good idea why.
Author:  shapenaji [ Wed Nov 24, 2010 1:47 am ]
Post subject:  Re: Physics puzzles
SpongeBob wrote:
cyclops, I liked your reasoning with the pressure, nice.

Well, it seems my first puzzle was not difficult enough for a smart croud like the one here on L19. Watch out: this one is REALLY cool, but also hard to figure out:

You have a ramp and you have two identical cans of soda. If you let both cans roll down the ramp, they will arive at the bottom of the ramp at the same time. Now if you shake one of the cans directly before the experiment, one of them will be faster than the other. Which one is it and why?
And if you do not believe that: try it out!

Well, here's my thought process.

What does shaking a can of soda do? It separates the carbonated bubbles from the solution. The liquid zone may loose a little volume and the "air zone" may gain a little volume along with a lot of increased pressure (The increased pressure shouldn't change the volume of the fluid additionally though,since the fluid is incompressible)

What effect does this have? It results in a change in the moment of inertia of the can, A transfer of mass takes place (an evening out if you will) between the top and the bottom of the can. This will result in a decrease. Which means that the shaken-up can should spin faster when given an equivalent amount of energy (in this case, gravitational potential energy)
Author:  entropi [ Wed Nov 24, 2010 2:38 am ]
Post subject:  Re: Physics puzzles
Made a try but deleted afterwards because it did not make much sense.

Here is another try:

What happens by shaking is that the mass inside the can is shifted from liquid to gas. The question is why it should make the can slower or faster?

While the can is rolling, the liquid moves inside, which creates friction. Since this requires some energy, it takes some portion of the potential energy that should normally turn into cinetic energy. Thus, the can with less liquid mass should be faster (assuming the friction depends on mass).

But, otoh I would guess that an empty can would roll slower than a full one, which is a contradiction to my reasoning.

Therefore my conclusion is: It is an inexplicable twilight zone phenomenon.
Author:  HermanHiddema [ Wed Nov 24, 2010 2:55 am ]
Post subject:  Re: Physics puzzles
The title "Physics puzzles" reminded me of this one:

Image

from xkcd :)
Author:  Tommie [ Wed Nov 24, 2010 3:21 am ]
Post subject:  Re: Physics puzzles - mosquito meets train
O.k.,
here comes my attempt for a nice contribution to this thread:
ImageImage

Late in the evening of a warm summer night, into the setting sun,
a very heavy freight train is pulled by a strong and heavy locomotive
onto its way to destination.

A tiny mosquito - may be it was an ephemerid ('one-day-fly' in German) - happened to hoover above the rails - when,
its honeycomb eyes blinded by the evening sun, it turns around and
WANG - its short life found an end.

There was a moment that the train (and perhaps insect?) stood still (had speed zero).
True or not?
_______________________________________________________________________________________________________________________
You know the 'Impulserhaltungssatz' (German for momentum conservation law (theorem) in physics)

momentum (SI unit kg·m/s, or, equivalently, N·s) is the product of the mass and velocity of an object (p = mv).

p = mv = constant,
resp. p1 x m1 = p2 x m2

The law of conservation of momentum states that if a body A of mass m1 and initial velocity u1 collides with a body B of mass m2 and initial velocity u2 , then after collision their final velocities will be v1 and v2 respectively:
(m1 x u1) + (m2 x u2) = (m1 x v1) + (m2 x v2) = constant

The question:
As i) the law is an axiom, hence must be true,
ii) the weight of neither heavy freight train nor tiny insect did change,
iii) speed of insect, above ground was initially about zero,
iv) speed of insect afterwards is = speed of train,
v) there was a time that insect had the speed zero
vi) then momentum = m(insect) x (speed = 0) = 0 =? momentum(train) is zero too?
vii) there is a first moment that insect & train have the same speed,
viii) hence speed v of both train and insect must have been zero at one moment?.
________________________________________________________________________________________________________________
EDITED: insect flies a tiny bit into the direction of the train
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