SEED Science

Floating and Stability: Buoyant Force

Find out more about
Floating and Stability

Center of Gravity 
General Stability
Buoyant Force
Floating and Sinking
Archimedes’ Principle
Center of Buoyancy
Stability and Center of Buoyancy
Stability in Sailboats

Buoyant Force

An object in a fluid is acted upon by an upward supporting force called the buoyant force. Sometimes this force can act as a restoring force to keep a floating object from tipping over. Let’s look first at the general idea of a buoyant force.

Have you ever picked up a heavy object underwater and brought it to the surface? If so, you may have noticed that the object seemed to be heavier when you lifted it out of the water. It was almost as if the water was helping to hold it up, as long as it was in the water. Let’s see how that could happen.

When an object is completely submerged in water, the water pushes against its sides as well as its top and bottom. At any specific depth, the force of the water pushing against the object’s sides is the same in all horizontal directions.

However, the force of the water pushing down on the object’s top is not as strong as the force of the water pushing up on the object’s bottom. This is because the water pressure increases with the water depth. The water depth at the top of the object is less than the water depth at the bottom of the object. Therefore, the water pressure acting on the bottom of the object is greater than the water pressure acting on the top.

Buoyant force

In the diagram above, imagine that the block-shaped object is submerged in water. The horizontal arrows labeled A represent the force of the water pushing on the four sides of the object. These forces are equally strong because they all are created by the pressure of the water at the same depth.

The arrows labeled B and C represent the force of the water pushing down on the top and on the bottom of the object. C is shown a little longer than B to indicate that the force from the water pushing up on the bottom of the object is stronger than the force from the water pushing down on the top of the object. The water pressure at the level of the bottom of the object is greater than that at the top of the object.

The difference between the smaller B force downward and the larger force C upward is a net upward force that works against gravity. This is the buoyant force that makes an object submerged in water seem less heavy than when it is out of the water.

This buoyant force is also the supporting force for an object that floats in water.