references : MOTION, FORCES AND ENERGY (MC GRAW HILL) NATIONAL GEOGRAPHY
THE BUOYANT FORCE
Think about the forces that are acting on you as you motionless on the surface of a pool or lake. You are not moving, so according to Newton’s second law of motion, the forces on you must be balanced. Earth’s gravity is pulling you downward, so an upward force must be balancing your weight.
WHAT CAUSES THE BUOYANT FORCE?
The buoyant force is caused by the pressure that is exerted by a fluid on an object in the fluid. The water exerts pressure everywhere over the surface of the object. Recall that the pressure exerted by a fluid has two properties. One is that the direction of the pressure on a surface is always perpendicular to the surface. The other is that the pressure exerted by a fluid increases as you go deeper into the fluid.
SINKING AND FLOATING
The buoyant force pushes an object in a fluid upward, but gravity pulls the object downward. If the weight of the object is greater than the buoyant force, the net force on the object is downward and it sinks. If the buoyant force is equal to the object’s weight, the forces are balanced and the object floats.
CHANGING THE BUOYANT FORCE
Buoyant force does depend on the shape of the object. The fluid exerts upward pressure on the entire lower surface of the object that is in contact with the fluid. If this surface is made larger, then more upward pressure is exerted on the object and the buoyant force is great.
According to Archimedes’ principle, increasing the weight of the water that is displaced increases the buoyant force.