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Elmo
04-10-02, 05:04 PM
I went on a visit to reading university recently, and a proffesor there set the following puzzle. The puzzle wasn't difficult itself, but the disscusion that followed the answer was quite interesting. If anyone was there or have seen the puzzle before just see what others make of it.

A monkey is sitting on a raft, in a swimming pool. On the raft with the monkey is a pile of bricks. The monkey wants to know what will happen to the water level if he starts chucking bricks into the water. Will it go up, down or stay the same?

c'est moi
04-10-02, 06:09 PM
i'll give it a shot

my first thought was that the bricks will now be IN the water, not on
this means that the bricks will cause the water level to rise cause of the added volume/brick
BUT, the bricks will also absorb water
maybe if they ly there a very very long time they'll absorb the volume of water which is *almost* equal to their own volum and so eventually the water level will have risen a little bit

Chagur
04-10-02, 06:32 PM
If the bricks don't float, the water level will go down.

If they float, it will remain the same.

A matter of different displacements; floating vs non-floating objects.

Take care ;)

Bigtraine
04-10-02, 10:47 PM
If the bricks don't float, wouldn't the water level increase? I would think that the bricks in the pool would displace the water and push it up, thus expanding and increasing the water level...

Chagur
04-11-02, 10:49 AM
Remember, when they were in the raft they were 'floating'.

Once shucked overboard, they sank to the bottom because they
displaced less water than was needed to keep them afloat.

Take care, and welcome to Sciforums :)

Elmo
04-11-02, 11:26 AM
So if the water level goes down, can you therefore empty the pool if you keep chucking bricks in? ;)

IggDawg
04-11-02, 12:00 PM
If the brick sinks, there is one brick-worth of volume displaced.

If the brick floats, there is one brick-worth of water mass displaced.

One brick-worth of water mass is more than one brick-worth of water volume since a brick is denser, so the level would go down.

And yeah, if I jam the swimming pool full of bricks it'll all overflow :D .

04-11-02, 12:20 PM
Anything with monkeys has to be cool. :)

Stryder
04-11-02, 01:01 PM
There is another point, what about the water level in relationship to the monkey. Afterall the monkey is sitting on a raft, so placing bricks in the swimming pool might raise the water in the pool, but the raft will still be floating on the surface.

Elmo
04-11-02, 04:14 PM
true, but the actual depth of the water would lower, if you put a mark on the side of the pool or something.

And yeah, if I jam the swimming pool full of bricks it'll all overflow

but the water level goes down. Obviously it can't keep going down beacause then with enough bricks you could empty the pool.
:bugeye: ;)

IggDawg
04-11-02, 04:26 PM
Yea, it'd keep going down and eventually collapse in on itself. Thats how black holes are made. monkeys and bricks. thats why you never throw a brick at a monkey, and at least try to avoid throwing monkeys at bricks (if possible). :D

Joeblow93132
04-12-02, 11:58 AM
Elmo,

I think the water level would remain the same.

After all, isn't the weight of the brick in the raft forcing more of raft to be submerged. Therefore, if you through the brick in the water the level would rise, but the raft would rise as well, causing the water level to fall the same amount.

Tom

04-12-02, 12:26 PM
Monkey on raft with bricks tosses bricks into the pool. Water level was already determined partly by the presence of raft, monkey, and bricks in the pool. However, with the bricks in the pool rather than on top pressing down, I would guess they might absorb some small amount of water, depends how porous they are, so a tiny fraction of the water which was simply displaced before to raise the water level can now occupy the same area as the bricks, or at least the holes in the bricks. So bricks in pool is lower than bricks on raft, by the tiniest bit.

That's my guess. :p

(Q)
04-12-02, 12:49 PM
The displacement is the weight of the water of the displaced volume of the raft, monkey and bricks; for static equilibrium it is the same as the weight of the raft, monkey and bricks. As well, the force of buoyancy is equal to the weight of the volume of water the raft, monkey and bricks displaces.

Therefore, if you put something in water and it floats, than it displaces its mass/weight. If an object in water sinks, than it displaces its volume.

Carry on...

Stryder
04-12-02, 01:49 PM
Perhaps the monkey doesn't understand what happens to the water, because it throws the bricks in that end up building this landbridge from the raft and then it escapes while running after a truck load of bananas that happened to be driving past.

(I could resist, as the logic of this topic is getting haphazard and spiralling on other peoples ideas, again and again... Argh!!!)

Okay I'm not that worked up, but I thought the thread needed a bit more humour.

Elmo
04-14-02, 04:11 PM
The question has been answered! Chagur, then Iggdawg then Q all got it, the level goes down. The question now is why can't you keep throwing bricks in and empty the pool.
I'll start you off. There's an assumption being made, does it allways hold. What is the assumption and when does it break down?

Chagur
04-14-02, 05:36 PM
Like maybe ... if you loaded enough bricks on the raft with the monkey,
you'd empty the pool before the monkey would throw any in?

Take care ;)