# Falling Marbles

Thick and gluggy versus thin and runny. Compete in a marble race and use different liquids to make your marbles move quickly and slowly.

## What you need

• 3 marbles (same size)
• water
• vegetable oil
• honey
• funnel
• 3 tall containers with lids, e.g. clear plastic tubes or glass jars (plastic tubes used to store buttons are ideal if the holes in the lids are sealed)

## What to do

1. Use the funnel to fill one container with water, the second with oil and the third with honey.
2. Drop a marble into each container and screw the lids on firmly.
3. Test how fast the marbles fall through each fluid by turning the containers upside down.
• Which marble falls the fastest?
• Can you think of a way to time the falling of the marbles?
• What could you do to the honey to make the marble fall faster?

## What's happening?

Viscosity is a measure of resistance to flow or how easily a fluid can change its shape. Water flows very easily so we say it has a low viscosity. Oil has a higher viscosity than water and honey has a very high viscosity at room temperature so these fluids tend to flow more slowly than water.

For a marble to fall through a fluid, the fluid has to change shape to let the marble through. High viscosity fluids have a lot of resistance to changing their shape so the marble takes longer to fall. Dropping a sphere in a liquid is one of the oldest and easiest ways to measure viscosity.

## Investigate!

Design an experiment to test how the viscosity of honey changes with temperature. This could involve measuring the temperature of the honey and timing the fall of the marble through the honey and repeating this for a number of different temperatures. You could then plot a graph showing the temperature on one axis and the time for the marble to fall on the other axis.

## Did you know?

The viscosity of volcanic lava is important in working out how it will behave and can be used to explain the shapes of volcanoes. Lava that has a high viscosity tends to flow slowly and is associated with highly explosive eruptions from steep, conical volcanoes. Lava with a lower viscosity flows quickly, forming channels and puddles and this type of lava arrises from quieter volcanoes that tend to have a broad, flat shape.