What makes a paper clip float?

Asked by: Timothy Hicks

Answer

A paper clip will not 'float' on water in the same sense that a cork will. If you position a cork below the surface, it will rise to the surface. A paper clip will not.

If you carefully place the paper clip on the surface of calm water, however, you can keep it from sinking because of surface tension. In a liquid, molecules are attracted by neighboring ones in all directions, except for those at the boundary. The unequal attraction of neighboring molecules at the surface forms a kind of 'skin'. This surface tension is responsible for minimizing the surface area of a free-falling drop of water into a sphere.

If you look closely at an object suspended by surface tension, whether it is a paper clip, insect, leaf, etc. you will notice a slight depression in the surface supporting the object. As long as the pressure exerted by an object is low enough, surface tension can support it. Low pressure requires a large area in contact with the water relative to weight.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor