Measurement procedure and experiment equipment

The device is a high thin transparent cylinder filled with an investigated liquid. At the top of the cylinder, there is a lid with an opening. We can drop a small-shot ball and observe its motion. It is not difficult to calculate the speed of the ball.

Friction force acts on a moving body in a viscous liquid. Amount of this force generally depends on many conditions such as liquid viscosity, a shape of a body etc. Stokes formulates the expression for the friction force F in case of a ball flowed round by unlimited laminar liquid:

, (6.9)

where h is a coefficient of internal friction; r is the radius of the ball; v is the speed of the ball.

Let us consider a ball dropped in a viscous liquid. There are three forces, which make the ball move (Fig. 6.3): 1) the gravity force:

, (6.10)

where V is the volume of the ball; r is the density of the ball; g is gravity acceleration;

2) the Archimedes force

(6.11)

where r₁ is the density of the liquid in the cylinder;

3) the force of internal friction F (see formula 6.9).