Cooking For Engineers

[OSUsammy13]

You are sitting in a boat with an elephant wearing scuba gear. The boat is in the middle of your standard backyard pool that just happens to be very deep. In your quest to be the last survivor, you push the elephant into the water and it sinks to the bottom (hence the scuba gear b.c we arnt cruel). If you had marked the level of the water in the pool before and after you pushed in the elephant, would the water level rise, fall or stay the same?
I am not sure how hard this problem really is, but non-engineers always get it wrong.

[capstinence]

My instincts say "rise," because I would think that the elephant would displace some of the water.

But my actual thought is that it would stay the same, since this is supposed to not be a blatantly obvious answer And I think I've heard the question before, but I cannot remember the actual reason why the water level would remain the same. Maybe because the elephant was in the boat already, meaning that the water was already displaced? It'd be different if you shoved him in from the side of the pool, yes?

[Chef Jim]

I agree the elephant had already displaced the water, however the act of pushing him in the water might create a splash, waves, etc. would there be significant loss of water from this action?
:?

[OSUsammy13]

The walls of the pool are high enough so that any splashes/waves are contained with zero water loss and so that if there is a rise in water level, it would be measurable.

Good Luck.

[OSUsammy13]

In order to help you along with your thinking, try to quantify your statements.

for instance, if you agree that the elephant had already displaced water, ask 'How much water'.
if the elephant is displacing water when he is pushed in ask 'how much water'.

[capstinence]

Am I right that the level doesn't change? I think I am.

[OSUsammy13]

I will message you privatly with an answer to this but you have not given a reason why it would stay the same.

[capstinence]

Except I already gave the reason in my first response.

After consulting an engineer (since that was the elitist preface to the puzzle), the water level goes down because, in the beginning, the elephant is displacing its weight, but once in the water is displacing its volume only.

This makes no sense to me, but I'm just an ignorant fiction writer.

[OSUsammy13]

Correct - the water level goes down. Congratulate your engineer friend.

[Michael Chu]

We've got to come up with a good way to post answers without spoiling the puzzle solving for others. Here's my reasoning:

Water in pool without elephant (just the boat) = Level 0
Water in pool with elephant in boat = Level 0 + X where X is the volume of water equivalent in weight to that of the elephant (this is true because the elephant in the boat is floating on the water - the water is holding up the elephant and the boat because the boat is displacing a volume of water that exactly matches the weight of the elephant and the boat).
Water in pool with sunken elephant = Level 0 + E where E is the volume of water displaced by the elephant.

Because the elephant sinks, the elephant must have a higher average density than the water. So, we know that the volume of the elephant is less than the volume of water of equivalent weight. So, E<X, and therefore, the water level lowers if the elephant sinks.

If the elephant floats, then the water displaced by the elephant is the same as when the elephant was in the boat and the water level will be the same.

Michael

[capstinence]

[futureboy]

This is a famous puzzle. (At least with a rock, not an elephant.) In Jearl Walker's excellent book of physics puzzles The Flying Circus of Physics, he mentions:

"This problem was asked of George Gamow, Robert Oppenheimer, and Felix Bloch, all excellent physicists, and to their embarrassment they all answered incorrectly."

[JohnLocke809]

[AdaWakeman]

Sorry for bumping this old thread but i saw that it doesn't have a certain answer in the end. And futureboy, what did George Gamow, Robert Oppenheimer, and Felix Bloch answered at the problem with the rock?
ADA engineer

[Michael Chu]

I believe my answer on May 18, 2005 was correct and I gave my thought process for it. You'll need to highlight the text in your browser to reveal it (it's in white text, so selecting it should make it readable).