Rock the Boat

Abstract

Level with rock in boat is  indicated by the blue line.
Level with rock in boat is
indicated by the blue line.

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?

Portable

Yes

Principles Illustrated

When the rock is thrown  overboard the level of the  pond goes down.
When the rock is thrown
overboard the level of the
pond goes down.

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.

Instructions

The boat and the rock.  Some additional mass  is added to the bottom  of the boat to keep  it upright in the water.
The boat and the rock.
Some additional mass
is added to the bottom
of the boat to keep
it upright in the water.

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.

Safety

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.

Related Resources

Archimedes Principle

Teaching Resources

Would you like to contribute lesson suggestions? Contact us.

Credits

This teaching resource was developed with support from

The MacDiarmid Institute
Faculty of Science, Victoria University of Wellington
School of Chemical and Physical Sciences, Victoria University of Wellington

Copyright

Copyright and fair use statement