Grades 3–5 · Materials & Matter
How can a tiny metal object rest on water without sinking?
A tiny metal object can sometimes rest at the surface of water even though a larger solid piece of that metal would sink. The water surface bends down around the small object. It is not a solid sheet, but it can help provide support.
In the diagramA bent water surface can help support an objectCross-section: a small nonwetting cylinderSupplied force account, not a size predictionSurface forceWeightBuoyancyUp: surface 8 + buoyancy 2 • down: weight 10Balanced record: zero net vertical forceForce units are invented; arrows are schematic.
Water molecules attract one another. Making more water surface requires energy, so the surface tends to contract. This effect is called surface tension. Around a suitably small object, forces along the sloping surface can have upward parts.
The water also pushes up through buoyancy. If the total upward support balances the object's downward weight, it can remain at the surface. The picture separates these contributions: two sloping surface forces and an upward buoyancy force oppose weight.
Support depends on the object's size, shape and contact with the water. A disturbed or contaminated surface can behave differently. This does not mean that water can hold any heavy object or that the metal has become less dense.
About this explanation
Starting knowledge: Recognize downward weight and an upward supporting force.
Sources
- Massachusetts Institute of Technology — Definition of surface tension
- Massachusetts Institute of Technology — Capillarity and gravity