Under
conditions of low velocity and non turbulent motion, the force
experience by a sphere moving through a fluid is described by Stoke's
Law:
A Level Physics Notes: Fluid Mechanics - Stoke's Law and Terminal Velocity
When a body moves through a fluid it experiences a resistive force because the the presence of the fluid. Every fluid has a property called viscosity – the resistance of a fluid to any body moving through it. Thicker, more viscous materials resist more the movement of bodies through them than thinner, less viscous materials.
The viscosity of a material is described by it's coefficient of
viscosity,Under
conditions of low velocity and non turbulent motion, the force
experience by a sphere moving through a fluid is described by Stoke's
Law:
For this case, of a sphere moving through a fluid, a free body diagram is shown below.
and
the density of the body is
the
mass of fluid displaced is
and
the weight of fluid displaced is
and
we can write the weight of the body similarly as
If
the body is travelling at it's terminal velocity all the forces
balance and
If we know the viscosity, the terminal velocity
is
given by
If we measure the terminal velocity, the viscosity
is
given by