An iron sewing needle laid flat can rest on a water surface even though iron is denser than water. Which statement describes what is happening?
AIt floats because it displaces enough water
BIt is held up by forces in the stretched surface
CIt floats because trapped air lowers its density
DIt is pushed up by a large buoyant force
Answer & Solution
Correct answer: B. It is held up by forces in the stretched surface
1. Floating requires an average density below that of the fluid, and iron is denser than water, so the needle cannot be floating.
2. What supports it is the water surface itself, which dents under the needle and pulls back like a stretched membrane.
3. Those restoring forces act along the surface and their upward components carry the needle's weight.
4. Stand the needle on its point and the same weight acts over a much smaller area, so the surface breaks and it sinks.
5. Calling this buoyancy or displacement misses the fact that a denser object cannot be supported that way.
_Source: OpenStax College Physics (CC BY 4.0), Ch 11 "Fluid Statics", section 11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action_
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