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.
2 · Watch
Take it off screen
Where it sits
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.