How does a heavy steel ship float?
A plain-English answer for grown-ups of kids aged 6–9, taken from the Yufena lesson that explores this question.
This is a members' lesson. Four other lessons are free to try.
The answer, for parents
Short answer: A wide, hollow hull pushes aside lots of water, and the water pushes back up hard enough to hold the ship.
Some things float on water. Some things sink. A steel ball will sink. Steel is heavy for its size. We say it has high density. A thing in water pushes some water aside. The water pushes back on it, upwards. This is the push up. The more water pushed aside, the bigger the push.
A ship's hull is wide and hollow, and full of air. So it pushes aside a lot of water. The push up gets big enough to hold up the whole ship. Think of the whole ship, steel and air together. All together, it is lighter than the same amount of water. So it floats. If water fills the hull, the air is gone. Then a ship can sink.
In the lesson, your child puts this cause-and-effect chain in order: A wide, hollow ship sits in the water → It pushes aside lots of water → The water pushes back up → The push up matches the ship's weight → The ship floats.
What if we load more cargo? The ship sits lower. It pushes aside more water, so the push up grows. But too much cargo is not safe.
Do ships float higher in the sea or a river? Sea water has salt, so it is a little denser. Ships float a bit higher in the sea than in a river.
Try it with your child
Let's build boats. Take two squares of foil. Squeeze one into a tight ball. Shape the other into a boat with high sides. Put both in a bowl of water. Now load your boat with coins, one at a time. How many can it carry? Watch how low it sits. What happens at the end?
Safety: Safety first: do this with a grown-up near the water. Dry up spills.
Explore it together in the full lesson
Guess, find clues, build the chain and try the experiment. Read it together.