http://www.ck12.org Chapter 15. Fluid Forces
Buoyant Force
Buoyancyis the ability of a fluid to exert an upward force on any object placed in the fluid. This upward force is
calledbuoyant force.
Pressure and Buoyant Force
What explains buoyant force? Recall from the earlier lesson "Pressure of Fluids" that a fluid exerts pressure in all
directions but the pressure is greater at greater depth. Therefore, the fluid below an object exerts greater force on the
object than the fluid above the object. This is illustrated inFigure15.12. Buoyant force explains why objects may
float in water. No doubt you’ve noticed, however, that some objects do not float in water. If buoyant force applies to
all objects in fluids, why do some objects sink instead of float? The answer has to do with their weight.
FIGURE 15.12
Fluid pressure exerts force on all sides of this object, but the force is
greater at the bottom of the object where the fluid is deeper.Weight and Buoyant Force
Weight is a measure of the force of gravity pulling down on an object. Buoyant force pushes up on an object. Weight
and buoyant force together determine whether an object sinks or floats. This is illustrated inFigure15.13.
- If an object’s weight is less than the buoyant force acting on the object, then the object floats. This is the
example on the left inFigure15.13. - If an object’s weight is greater than the buoyant force acting on the object, then the object sinks. This is the
example on the right inFigure15.13.
FIGURE 15.13
Whether an object sinks or floats depends on its weight and the strength
of the buoyant force acting on it.