4.Main Content

experiments

Falling through a high viscosity liquid

Demonstration

The higher the viscosity of a liquid the more it resists motion of a body through it. The result can be very low terminal velocity.

Apparatus and materials

• Measuring cylinder or tall and fairly wide glass tube, 1,000 ml, with firm stopper
• Glycerine, heavy oil or liquid detergent
• Ball bearings (approximately 3 mm and 1.5 mm)
• Chinagraph pencil, water-based pen, or elastic bands
• Eye protection

Technical notes

A tall glass tube allows a greater distance of fall than a measuring cylinder. Seal the bottom end firmly with a stopper and rest this on a surface so that it cannot fall out. Do not over-tighten any clamp that you use to hold such a tube.
 
Use the pencil, pen or elastic bands to provide equally spaced markers on the measuring cylinder or glass tube. Do this before the lesson.
 
Place the ball-bearings in a dish of the same liquid before use. This reduces the occurrence of air bubbles, which will affect the motion of the ball bearings.
 
Retrieve ball-bearings from the liquid with a magnet outside the jar. This is a messy activity to clear away, especially if many ball bearings are allowed to fall and must then be retrieved.

Safety

Glycerine (glycerol or propane -1, 2, 3-triol) will irritate eyes, so eye protection should be worn. Its properties will change if it is allowed to absorb water vapour from the atmosphere so it must be kept in a closed container. Waste engine oil is carcinogenic and must not be used.

Read our standard health & safety guidance

Procedure

a Set up the measuring cylinder or tall glass tube, filled with the viscous liquid, so that it is illuminated from above by a bright source. In an otherwise darkened room (full blackout is not necessary) the ball-bearings then appear as bright points of light.
 

 
b Release a ball-bearing from just above the liquid surface.
 
c Ask students to clap as the ball-bearing passes each marker. This is sufficient to show that the time intervals become the same, and thus that the ball-bearings quickly reach their terminal velocity.

Teaching notes

1 You could use a more sophisticated timing system, but the point here is to demonstrate terminal velocity rather than to make precise measurements.
 
2 Advanced level students could determine the viscosity of the liquid, using Stokes' law. Or they could investigate the relationship between the radius of a falling ball and its terminal velocity. When a ball bearing is moving at terminal velocity, the forces acting on it are balanced.
 
Frictional force acting upwards = weight - upthrust
 

 
where η = viscosity
α = radius of the ball bearing
ν _ο = terminal velocity
γ = gravitational field strength
ρ = density of the bearing material
σ = density of the liquid
 
This experiment was safety-checked in April 2006