Physics puzzles

[SpongeBob]

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 : 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?

[wms]

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.

[Li Kao]

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.

[Koroviev]

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.

[wms]

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?

[cyclops]

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.

[jts]

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

[wms]

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.

[DrStraw]

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?

[entropi]

Another one: you forget the refrigarator open. (Why) Will the room temperature increase or decrease?

[Sverre]

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.

[cyclops]

Question for entropi. How much of you is on a [font=]green[/font] photon.

[entropi]

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.

[cyclops]

Photon entropy is independent of color

Is it? Then it might be k, the Bolzmann constant, 1.380 6504(24) × 10−23 J/K.

[SpongeBob]

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!