What it shows
A ball falling in a thick liquid reaches a steady speed where the drag balances its weight less the buoyancy. Stokes’s law then gives the liquid’s viscosity from that speed.
The science
When the ball falls at a steady terminal speed v, weight = buoyancy + viscous drag: (4/3)πr³ ρs g = (4/3)πr³ ρl g + 6π η r v. This gives the viscosity η = 2 r² (ρs − ρl) g / (9 v). Stokes’s law holds when the flow is smooth, which needs a slow fall, and a jar much wider than the ball. Viscosity falls rapidly when the liquid is warmed, so the ball falls faster in warm honey.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A tall clear jar or measuring cylinder (about 30 cm) filled with honey or glycerine
- A glass marble or a steel bead of known diameter (measure it with a ruler against the jar or a vernier calliper)
- Marks of tape on the jar 5 cm apart
- A phone stopwatch or video
- A kitchen scale to weigh the marble, and a cup to find the liquid’s density
- A thermometer
- Warm water in a bowl to change the temperature
Safety
For this project: An adult must supervise any warming of the jar in a bowl of hot water; use warm, not hot, water.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Weigh the marble and measure its diameter to find its radius r and its density ρs (mass over (4/3)πr³).
Find the density of the honey by weighing a known volume, for example 100 mL, on the scale.
Stick tape marks down the jar and note the room temperature of the honey.
Release the marble at the surface of the honey, centred in the jar, and time it between two marks far below the top where its speed has become steady.
Repeat five times and average; calculate v = distance / time.
Compute η = 2 r² (ρs − ρl) g / (9 v) in pascal seconds.
Warm the jar in a bowl of warm water to 35 or 40 degrees C and repeat the measurement; compare the viscosities.
What you should see
Honey’s viscosity at room temperature comes out at several pascal seconds, thousands of times that of water, and it is clearly lower when the honey is warmed.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is viscosity?
The internal friction of a fluid, that resists the relative motion of its layers.
2What is terminal velocity?
The constant speed reached when the drag and buoyancy balance the weight.
3How does temperature change the viscosity of a liquid?
It falls as the temperature rises for liquids.
4Why is the speed measured far below the surface?
The ball needs time to reach its steady speed.
Take it further
Compare mustard oil, coconut oil and glycerine at the same temperature and rank their viscosities.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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