What it shows
Viscosity is how much a liquid resists flowing, and it falls quickly as the liquid warms. Timing a marble falling through honey or oil at different temperatures lets you measure this.
The science
As a marble falls through a liquid, a drag force from the liquid opposes its weight, and the marble quickly reaches a steady speed called terminal velocity. A thicker, more viscous liquid gives more drag and a slower speed. Warming makes the molecules slide past each other more easily, so the viscosity drops.
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 glass bottle or jar
- Honey, cooking oil and shampoo, one jar each
- A marble or steel ball bearing
- A stopwatch
- A ruler and a marker
- A bowl of warm water at 50 C and a bowl of ice (adult prepares)
- A thermometer
Safety
For this project: An adult must prepare the hot water. Do not use microwave heating on glass jars, and wash up carefully because honey and oil make floors slippery.
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
8 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.
Fill the jar with honey and mark two lines 10 cm apart in the middle of the jar.
Drop the marble from the surface and time how long it takes to travel between the two lines.
Repeat three times and take the average, and work out the speed in cm per second.
Stand the jar of honey in a bowl of warm water at about 40 C for fifteen minutes.
Take its temperature, then drop the marble and time it again.
Repeat with cold honey from a bowl of ice water.
Do the same set for cooking oil and shampoo.
Draw a graph of speed against temperature for the liquids.
What you should see
The marble falls slowest in cold honey and fastest in warm oil, and speed increases as the temperature rises. The graph shows how temperature changes viscosity.
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 resistance of a liquid to flow.
2Why does warmth lower it?
Molecules move more freely when they have more energy.
3What is terminal velocity?
The steady speed when drag equals weight.
4Which flowed fastest at room temperature?
Normally the cooking oil, then shampoo, then honey.
Take it further
Use two marble sizes and find how the speed depends on the marble radius.
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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