How can a paperclip float on the surface of water?
1 Answer
Molecules of the same liquid (like water) are attracted to each other, which is a manifestation of cohesive forces.
Molecules inside the body of water are surrounded by similar molecules from all sides, so the cohesive forces are pulling a molecule equally in all directions.
All molecules on the surface of water have neighbors only inside the water and none on the outside. So, the balance of forces is not equal to zero and all the molecules on the surface are simultaneously pulled inside, which creates a tension on the surface. The surface behaves similarly to an elastic layer.
The surface tension is the result of rain droplets to take a shape of a sphere because a sphere is a shape with the smallest area of surface with a given volume. Molecules on the surface are equally squeezed inside, thus tending to diminish the area of a droplet.
With a paperclip situation is similar. If it is put accurately on the surface of water, it pushes down the surface molecules, thus changing the shape from flat to crater-like. But flat surface has smaller area than a crater, and the surface molecules tend to take a shape of a smallest area possible, so the surface tension is a force directing upward to flatten the surface. Apparently, these forces are sufficient to hold the paperclip on the surface. Obviously, the heavier the paperclip - the less chances surface tension has to hold it afloat.
