Science: An Elementary Teacher’s Guide/Water Pressure and Buoyancy

Pressure and Buoyancy
Pressure, which is a force exerted upon an object over a given area of space, affects buoyancy when the object is submerged. An object submerged in water will experience a buoyant force equal to the weight of the water that is being displaced by the submersion.



An object will float if the gravitational (downward) force is less than the buoyancy (upward) force. So, in other words, an object will float if it weighs less than the amount of water it displaces. This explains why a rock will sink while a huge boat will float. The rock is heavy, but it displaces only a little water.

Why do ships float on water? Ships float because they displace enough water for the buoyancy force to be greater than the force of gravity acting on the ship. The displaced water attempts to return to its original position, now occupied by the ship, which pushes the ship up. This effect is known as buoyancy force. The strength of the force of gravity pulling down on the ship is affected by its weight. Whichever of these forces is stronger determines if the ship floats or sinks.



How do ships float?