Floating, Sinking and Upthrust
Water pushes up on anything placed in it, and whether an object floats depends on that upward push against its weight rather than on how heavy it feels in the hand. A heavy steel boat floats and a small steel nail sinks, which is the puzzle this stop settles.
What a learner can do afterwards
- Predicts whether an object will float and gives the upward push from the water as the reason
- Explains why a large hollow object can float when a small solid one of the same material sinks
- Feels and describes an object becoming lighter as it is lowered into water, and names that change as upthrust
1 · Read
Water pushes up on everything in it, and the push is called upthrust. Your arms feel light in the bath because the water holds you up.
Sink or float is a contest between the push and the weight. If the push matches the weight, the thing floats. If the weight wins, it sinks, but the water holds back part of it.
A giant steel ship floats while a tiny steel nail sinks. The ship is hollow, so ship plus air are light for their size. The nail is solid, so it stays heavy for its size. Shape matters as much as stuff. The wide bowl pushes aside more water, so it earns a bigger push.
Try it in the bath with a small ball. Lower it slowly and feel the push grow as more of it goes under. Things that pack little weight into their size float, and things packed tight sink. For a floating ball, the push keeps growing until it matches the weight.
Water pushes up on everything, so what is light for its size floats and what is packed tight sinks.
The rewritten readings were written for this dataset.
More to read
Water's upward push
Upthrust is water's upward push on an object. Your arms feel lighter in a bath because of this push. They feel heavier once you get out. The push equals the weight of the water moved aside. When upthrust balances weight, an object can stay afloat or rest below the surface. If weight pulls harder, the object sinks, although water still supports part of its weight.
Understanding density
A heavy log can float while a light coin sinks. Density compares mass, the amount of matter, with the space it fills. For their sizes, the log is light and the coin is heavy. An object's average density below water's lets it float. Above water's, it sinks. Oil stays on top because it is less dense than water.
15.2: Buoyancy
How can a giant steel ship float when a tiny steel nail sinks? The ship cheats by being hollow. Its big belly holds air, so ship plus air together are light for their size. The nail is solid steel all through, so it stays heavy for its size. Shape matters as much as stuff. Squash clay into a ball and it drops. Spread the same clay into a wide bowl and it rides the waves.
2 · Watch
Take it off screen
Where it sits
Learn first
Optional connections
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.