Buoyancy
Summary
- Buoyancy (also known as the buoyant force) is the force exerted on an object that is wholly or partly immersed in a fluid.
- The symbol for the magnitude of buoyancy is B or FB
- As a vector it must be stated with both magnitude and direction.
- Buoyancy acts upward for the kind of situations encountered in everyday experience.
- As with other forces, the SI unit of buoyancy is the newton [N].
- Buoyancy is caused by differences in pressure acting on opposite sides of an object immersed in a static fluid.
- A typical situation:
- The pressure on the bottom of an object is greater than the top (since pressure increases with depth).
- The force on the bottom pushes up and the force on the top pushes down (since force is normal to the surface).
- The direction of the net force due to the fluid is upward.
- Pressure variations in a fluid are typically caused by gravity (since P = P0 + ρgh), but in general buoyant forces act opposite the direction of the frame of reference acceleration.
- Under conditions of apparent weightlessness there can be no buoyant forces.
- A typical situation:
- Archimedes' Principle
- The magnitude of the buoyant force on an object is equal to the weight of the fluid it displaces.
B = ρgVdisplaced
- The factors that affect buoyancy are…
- the density of the fluid
- the volume of the fluid displaced
- the local acceleration due to gravity
- The buoyant force is not affected by…
- the mass of the immersed object
- the density of the immersed object
- Objects immersed in a fluid have an apparent weight that is…
- reduced by the buoyant force (less than their actual weight)
W′ = W − B (W′ < W)
- directly proportional to the relative density (ρ′ = ρobject − ρfluid)
W′ = ρ′gV
Buoyancy and density densities B > Wobject B = Wobject B < Wobject ρobject < ρfluid object rises
(wholly immersed )float on surface
(partly immersed)ρobject = ρfluid neutral buoyancy
(wholly immersed)ρobject > ρfluid object sinks - reduced by the buoyant force (less than their actual weight)