# Measuring the radius of a marble

##### Class practical

Controlling the object you are measuring.

#### Apparatus and materials

*For each student group*

Metre rules, 2

Ruler, 15 or 30 cm

Set squares, 2

Marbles, a supply of

Supply of ball bearings of different sizes (at least 5 of each size) OPTIONAL

#### Health & Safety

Care needs to be taken as there is a trip hazard if marbles fall and roll around the floor.

#### Procedure

**a** Give students a single marble and ask them to measure the radius using a ruler.

**b** Discuss with the class the difficulties with this method, in particular the shape of the marble.

**c** Give the students the opportunity to devise an improvement before suggesting the method illustrated above. Two metre rules provide a trough which the marbles sit in so that they do not roll away. The rules are held next to each other and the pair of set squares makes the ends of the line of marbles clear.

**d** Students then can carry out this revised and improved measurement of the marbles and work out the radius of one.

#### Teaching notes

**1** In step **a**, students should be able to get a reading but the variation across the class may be considerable. Check that they have not given the marble diameter as their result, instead of its radius.

**2** This experiment is designed to support the development of practical skills and confidence, not simply finding the radius itself. Students may ask 'how many marbles should we use?', to which a non-committal reply such as 'how many do you think' or 'enough' is appropriate. Using increasing numbers of marbles reduces the uncertainty in the diameter measured, but there is no right number. It is worth mentioning that errors may be introduced when more marbles are used, e.g. not all the marbles may be in a perfectly straight line.

**3 How Science Works extension:** This experiment can support the skills involved in devising experimental methods which improve the accuracy of measurements. If students have carried out the experiment Measuring paper or Measuring the thickness of a coin, then they will already have encountered some of these ideas. In both of those experiments, a micrometer can be used to collect an accurate answer from a single coin or piece of paper.

*This experiment was safety-checked in December 2006*