Have you ever been on a boat cruise and wondered how you’re staying afloat? As you watched a tugboat chugging along, have you thought of why that heavy hauler isn’t on the bottom of the harbour? How are these ships able to float on water, which is much less dense than steel?

We can thank Archimedes, a Greek scientist who lived in the third century BCE, for being the first to explain the principle behind this perplexing pelagic puzzlement. This principle is known as buoyancy or Archimedes’ Principle.

Did you know? Archimedes’ Principle states that the force exerted on an object in a fluid is equal to the weight of fluid displaced by the object.

This force exerted on the object in a fluid is called the "buoyant force". It acts in all directions, but has an overall upward direction. Gravity exerts a downward force on the object, which is determined by the object’s weight.

Did you know? If the force exerted downward on the object by gravity is less than the buoyant force the object will float.

If a block of wood measuring one cubic centimeter is placed in a container of water, the amount of water displaced will equal the weight of the block of wood. However, if a one cubic centimeter block of a denser material, such as lead, is placed in a container of water, the amount of water displaced will equal the weight of the block of lead. In the case of the wood, the weight of the water displaced is small. The buoyant force is greater than the gravitational force so the wood floats. The lead is denser than the wood, meaning it contains more mass in the same volume (one cubic centimeter). More water is displaced by the lead than the wood. The gravitational force on the lead exceeds the buoyant force, thus the lead sinks.

How can this principle be applied to boats? Boats are shaped very differently from a solid block of wood or lead. Their hollowed out shape means a large portion of the interior of a boat is air, which is much less dense than water. The combination of the steel and air in a boat is actually less dense than the water it displaces. Therefore, a boat will displace water equivalent to its own weight before the boat is completely submerged. Eureka! That is how boats are able to float.

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Article first published on November 18, 2010.

Photo Credit:Wikimedia Commons

Amy MacDonald

I am a postdoctoral fellow at the Alberta Centre for Toxicology located in the University of Calgary's Faculty of Medicine. My Ph.D. is in analytical chemistry and I am really enjoying applying that knowledge to the field of toxicology. In my free time I enjoy doing science outreach, running (training for a 10K right now), playing softball, and reading.

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