Measure accurately

How can I teach students to measure accurately?

Length

Rulers, tape measures and trundle wheels are suitable, depending on the length being measured. It may be possible to get your scientist mentor to bring in vernier calipers or a micrometer to show how very small distances can be accurately measured.

Key points to focus on when measuring length are:

• Ensure that students carefully align the object being measured with the ruler and their eye that is. ensure that they are all in a straight line. When measuring each others’ height students need to be elevated or bend their knees so as to get their eye, ruler and top of the subject’s head aligned.

• Have students tabulate results to illustrate the variation in readings when taken from different angles that is. when eye, ruler and object are NOT aligned.

Volume

Volumes of liquids can be measured in measuring jugs and cups. Rain gauges can be used to practice measuring liquids with a scale.

Measuring cylinders, because of their shape and scale and the way they have been carefully calibrated can give more accurate measurements of volume than everyday items such as measuring jugs. The volume of small, irregular solids can be obtained by how much water is displaced when it is submerged in water. This type of method is frequently used when measuring lung volume by students expelling the air in their lungs into a large inverted container full of water.

To record the volume of an irregular solid, firstly record the volume of a liquid in a volumetric container and then add the solid. The volume will increase by the volume of the irregular solid. Alternatively there is the ‘overflow’ method. Students can measure the volume of their hand by immersing their hand in a full container that has a device to collect the overflow. The volume of the liquid that overflows is the same as the volume of their hand. (Read ‘Mr Archimedes’ Bath’ by Pamela Allen)

Students can investigate error in measurement of volume by adding 20 x 5 mL teaspoons of water to a 100 mL measuring cylinder. The fact that the reading is not exactly 100 mL is a reflection of how small errors, if repeated, will have an impact on accuracy.

Mass/Weight

Scientists distinguish mass from weight. Mass is the amount of matter, measured in kilograms (kg) and is a constant throughout the universe. Weight is the force on that mass that results from gravity and is measured in newtons (N).The weight of an object will vary with the strength of gravity. You will weigh less in the low gravity of the moon that you do on earth, but your mass remains the same.

In everyday non-scientific language we talk about an object’s weight (for example, 1 kg of sugar) when in fact we are referring to its mass. One kilogram of sugar actually has a weight of 9.8 N on Earth.

We measure mass using a balance (like the justice scales) where exact masses are placed until a balance is achieved. Generally for primary students, using bathroom and kitchen scales that record a mass are sufficient.

Time

Time is an ideal dimension for discussing digital versus analogue measurements. Students can use a range of clocks and stop watches depending on the required accuracy of the measurement.

Students will need to be taught how to read and record digital time measurements. Even young children are able to do this although their conceptual understanding of tenths and hundredths of seconds is likely to be underdeveloped.

As timing of short time intervals with a stop watch depends on reflexes, errors can be significant. It is important to pilot investigations and look at the variation of results. If students are measuring how long it takes a ball to fall through a specific distance, there can be errors in both starting and stopping the stopwatch. A pilot investigation may suggest you need to modify the procedure for example, drop the ball from a much greater height so that variations in results are not significant compared to the overall reading.