Physics: Drops of Water on a Penny


Due to the forces of surface tension, (Surface tension is a property of the surface of a liquid that allows it to resist an external force.) it is possible to place up to 36 or more drops of water onto a US penny, without the water spilling off the sides of the penny. The volume of water able to be placed on a penny vastly exceeds the volume of the actual penny.

The cohesive forces among the liquid molecules are responsible for this phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area.

Other examples of surface tension include:  the floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) and even reptiles (basilisk) to run on the water surface. This property is caused by cohesion of like molecules, and is responsible for many of the behaviors of liquids.

Sources: Random Science FactsWikipedia