A person sits in a boat on a pond holding a rock. If she throws the rock in the water, does the level of the pond go up, stay the same, or drop?
The water level of the pond drops! Surprised? Here’s why the water level drops: When the rock sits on the bottom of the lake it displaces its own volume. But sitting in the boat it displaces its weight in water. The density of rock is greater than the density of water, so the rock displaces more than its own volume when it sits in the boat and thus raises the water level of the pond more than it does when sitting on the floor of the pond.
NCEA & Science Curriculum
Fluids are not part of the NCEA physics curriculum but this is a great logic problem and it makes a great extension.
The top of a drink bottle makes a good boat. Put a bit of mass in the top of the bottle (bottom of the boat) to keep it upright, and then add the rock. We uses 100 g of brass in the photos above for a rock. Mark the level on the pond and then remove the rock and put it in the pond. The level goes down. This is a good demonstration for use with clickers and peer instruction.
Be careful with electronics around water.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
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