Understanding Buoyancy

• Upthrust = Weight of Displaced Fluid = Volume x Density
  • So it is about relative density
• If the bottom has no pressure to form a difference, there is no buoyancy. For example, in suction cups.
• But why is the water at the bottom pushing upwards?
  • Pressure is the volumetric energy of the particles in the fluid
  • Technically, you can apply this to solids too, as energy per unit volume is the normal force per unit area.
  • For air, the energy is due to the collisions of the number of particles, due to compression increasing the number of particles per unit volume
  • For fluids and solids, the energy due to the restorative forces, acting like compressed springs, due to incompressibility
  • Because of the energy, it exerts a force on the body, pushing on it in the normal direction(s)