Can a penny float on water? While it typically sinks, a fascinating scientific explanation reveals it’s possible with careful technique. This surprising outcome involves understanding fundamental properties of both the penny and water.
Why Pennies Usually Sink
A penny normally sinks due to density. Density measures how much mass is in a given volume; for an object to float, its density must be less than the fluid it’s in. A U.S. penny has a density of approximately 7.15 to 7.2 grams per cubic milliliter (g/mL).
Water, conversely, has a density of about 1 gram per cubic milliliter (g/mL). Since the penny is significantly denser than water, its weight’s downward force is much greater than the water’s upward buoyant force. Buoyancy is the upward force that a fluid exerts on an immersed object. Therefore, the penny displaces an amount of water that weighs less than the penny itself, causing it to sink.
The Role of Surface Tension
The ability to float a penny relies on a different property of water called surface tension. Water molecules are strongly attracted to each other through cohesive forces, particularly hydrogen bonds. Molecules within the main body of the water are pulled equally in all directions by their neighbors. However, molecules at the very surface of the water lack neighboring molecules above them. This imbalance results in an inward pull on the surface molecules, causing the water’s surface to behave like a thin, stretched elastic film or “skin.”
This film resists external forces and tries to minimize the surface area of the liquid. Water exhibits a notably high surface tension, approximately 72 millinewtons per meter at 20°C. This property allows light objects or those with a broad, flat surface to rest upon the water without breaking through.
Making a Penny Float
To make a penny float, carefully place it on the water’s surface without disrupting this delicate surface tension. The goal is to avoid breaking the cohesive bonds that form the water’s “skin.” A common method involves gently lowering the penny flat onto the water. Using a fork or a bent paperclip can help in this process by providing a stable platform to support the penny as it approaches the water.
The penny needs to be lowered slowly and horizontally, allowing the water’s surface tension to deform slightly around its edges without rupturing. If successful, the water’s surface will visibly curve or form a shallow depression around the penny, supporting its weight. The water essentially creates a temporary, flexible hammock, demonstrating how surface tension can overcome the force of gravity for objects that would otherwise sink.