The Concept:
Buoyancy
Buoyancy is the force acting on an object that is placed into water. If an object has a buoyancy force acting on it that is greater than the weight of the object, the object will float in the water. If the object has a buoyancy force acting on it that is less than the weight of the object, the object will sink in the water. If the forces are equal the, object will remain at any level of water. Fish are an example of something that has equal forces acting on it.
The actual buoyancy force is determined by how much water an object displaces. The volume of water an object displaces is equal to the mass of water the object displaces. Using Newton's 2nd Law of Motion, the buoyancy force can then be calculated by using this mass of water. The units on force will be Newtons (N), the units on the mass of water kilograms (kg), the units on acceleration are meters per second squared (m/s2).
Buoyancy Force (Bf) = mass of water (m) x acceleration due to gravity (a)
Bf = m x a
Pressure
Pressure and force are related concepts put are in fact different from one another. Pressure is the amount of force acting on an object over a unit area. Pressure is usually listed in Pascals (Pa) which is the same as a Newton per square meter (N/m2). Force is given in Newtons (N), and areas are given in square meters (m2).
Pressure = Force Area
P = F A
Imagine a rectangular object such as a shoe box. The shoe box has the same mass regardless of if you set it on any of its sides. However the pressure on the box will change depending on the side it is placed on. Let’s do a sample calculation to show this. If the box had a mass of 1.50kg, it would be exerting a total force of 14.7 Newtons. Now assume the box has sides of 0.10 meters, 0.20 meters, and 0.30 meters. This would give it sides with three different surface areas of 0.20 square meters, 0.30 square meters, 0.60 square meters. Using the above the different surface areas give values of 73.5, 49.0, and 24.5 Newtons per square meter (N/m2) for the different pressures on the shoe box. This shows how changing the surface area affects the amount of pressure on the side of the box touching the ground.