A nail sinks. A ship made of the same kind of stuff floats. What changed?
Eve can say this.
A nail sinks. A ship made of the same kind of stuff floats. What changed? Make a foil boat. Then crumple the same foil into a ball.
Words Eve says
A nail sinks. A ship made of the same kind of stuff floats. What changed? Make a foil boat. Then crumple the same foil into a ball.
A sink, foil, and your hands
Use a sink or a tub. No need for real nails, and no standing water left unattended with little siblings.
Press a square of foil into a little boat. Set it on water. It should sit.
Now crumple the same foil into a tight ball. Same foil. It sinks.
You changed one thing: the space it takes up.
What if the boat had a hole?
Look closer
Notice
The material did not become lighter. Steel is still steel.
The ship takes up more space. It pushes a lot of water out of the way.
Water has weight. When you push water aside, the water pushes back.
What do you think will happen?
Take one lump of clay. Ball it. Then press it into a boat. Same clay. What happens?
Change shape: ball or boat, same amount of stuff.
Keep looking
What must be true?
Water pushes up with a force equal to the weight of the water you push aside.
A ball of clay pushes aside a little water. That upward push is smaller than the clay’s weight. It sinks.
A thin boat of the same clay pushes aside much more water. When the upward push matches the weight, it floats.
The upward push has a name.
Buoyancy is the water’s push. Density is how much stuff is packed into a space. A ship’s steel is dense. The ship as a whole, steel plus the air inside the hull, is less dense than water. The nail has no hollow. It cannot push aside enough water to match its weight.
A nail sinks. A ship made of the same kind of stuff floats. The material did not become lighter. The ship takes up more space, so it pushes more water aside. Water has weight. The water you push aside pushes back. When that upward push matches the weight, the ship sits on the water. Try a ball of foil. Then open it into a boat.
Words Eve says
A nail sinks. A ship made of the same kind of stuff floats. The material did not become lighter. The ship takes up more space, so it pushes more water aside. Water has weight. The water you push aside pushes back. When that upward push matches the weight, the ship sits on the water. Try a ball of foil. Then open it into a boat.
The bath, and what the story changed
A story says Archimedes sat in a bath in Syracuse, watched the water rise, and ran into the street.
The later retellings added the running and the shouting. What we can keep is the idea: the water you push aside has weight, and that weight is the upward push.
Shipwrights already knew, in their hands, that a hollow hull sits on water. The principle named why.
- Britannica: Archimedes’ principle · The buoyant force equals the weight of the displaced fluid.
- NASA Glenn: Buoyant force · An object floats when the weight of displaced fluid matches the object’s weight.