What Makes an Object Float in Water?

For an object to float in water a key condition has to be met. The volume of water displaced by the object must weigh the same as the object. The principle involved is known as Archimedes' principle. Though it may be a bit of a tricky concept at first, a simple example serves to illustrate the principle.

Consider a mass of steel, say, the size and shape of a bowling ball. When placed in water it will sink. But the same mass of steel made into the shape of a bowl, when placed in water so that the concave side of the bowl is facing up, will float. This example satisfies the requirements of Archimedes' principle.

In the case of the steel being in the shape of the solid bowling ball, the volume of water displaced does not weigh as much as the mass of steel and this results in the ball penetrating down into the water, becoming completely submerged and sinking. The volume of water displaced is identical to the volume of the steel ball, but since that volume of water weighs less than the steel ball, the ball sinks.

In the case of the same mass of steel being shaped into the form of a bowl and placed on the water's surface, the amount of water displaced is more than that of the volume of the solid steel ball. The bowl-shaped steel displaces enough water to equal the weight of the steel. This, of course, explains how and why large, heavy ships made of steel can float. If they were in the shape of a block, a ball or some other shape that permitted them to displace less water, they would sink.

Archimedes' principle applies to any object in water. Objects either float or sink based upon whether the weight of water they displace is less than or equal to their own weight. The one-word term used to represent Archimedes' principle is buoyancy.

References

• Weber State University, Archimedes' Principle

Resources

• Buoyancy