Homework Help: Displacement of Water

1. Nov 9, 2003

TigerLilly

My friend and I are arguing over this question. If you're in a boat on a pond and drop a large rock into the pond will the water level change? If so will it increase or decrease? What will happen if you catch a fish that has swallowed a lot of water? I said that the rock will increase the water level b/c it's like dropping pennies in a cup full of water, is displaces the same amout of water that's equal to the mass of the penny. In the case of the fish that water is taken out so the level decreases. The difference is very small though. My friend says that the water level will stay the same. Who's right?

2. Nov 9, 2003

Ambitwistor

You're both wrong: the water level goes down if you drop a rock over the side. Don't feel bad; this famous puzzle has tripped up a lot of people. See:

(the solution is on pages 2 and 3)

http://www.rain.org/~mkummel/stumpers/07sep01a.html

Last edited by a moderator: May 1, 2017
3. Nov 9, 2003

gnome

Neither of you.

When the rock is in the boat (which is floating), it displaces its weight in water.

When it is thrown in & sinks, it displaces its volume of water.

Since the rock weighs more than an equivalent volume of water, it is displacing more water when floating than when it is at the bottom of the lake. So when it is thrown in, the level of the lake falls.

4. Nov 9, 2003

TigerLilly

What happens then when a fish is caught that has swallowed a lot of water?

5. Nov 9, 2003

Ambitwistor

If you mean the fish is denser than water, then it's equivalent to a rock: pulling it out of the water will be the reverse of throwing it in, so the water level will increase.