Pressure and floating
Pressure is force spread over the area it acts on. The same force on less area gives more pressure, which is why a nail goes in point-first, a sharp knife cuts, and heels mark a floor more than an elephant. Under water pressure grows with depth, and depends not on how much water is around you but on how much is above you. And everything submerged gets an upward push equal to the weight of the water it displaces, which is Archimedes' principle.
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Exercises from this rung
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Why does a nail go in point-first and not head-first?
- Because the point touches less area
- Because the metal is harder there
- Because the point weighs less
- Because the point moves faster
Why The same force spread over a tiny area gives an enormous pressure. The point does not push harder: it pushes the same, but concentrated. It is the same reason a sharp knife cuts and a blunt one does not, without either weighing more.
easierHow much pressure is there 30 m under water, counting the atmosphere?
Answer 4
Why One atmosphere from the air above plus 3 from the water: 4 atm. Pressure does not depend on how much water is around you but on how much is **above** you, which is why a small lake and the ocean squeeze the same at the same depth.
in betweenWhat is the upward push on a submerged body equal to?
- Its volume
- The weight of the body
- The depth
- The weight of the water it displaces
Why The weight of the water that took its place, no more and no less. If that weight beats the body's own, it floats. That is why a steel ship floats and a steel ball sinks: the ship encloses air and displaces far more water with the same mass.
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