Making better measurements

In Science, we often make measurements. We do this to find out more about something that we are interested in.
Measurements are made using measuring instruments. These include rulers, balances, timers and so on.
We want our measurements to be as accurate as possible. In other words, we want them to be as close as possible to the true answer. Then we can be more confident that our conclusions are correct.

Measuring instruments

How can we be sure that our measurements are as accurate as possible? We need to think about the instruments we use. Here are two examples:
• You want to measure a $50cm$ volume of water. It is better to use a $100cm$ measuring cylinder than a $50cm$ beaker, even though the beaker may have a line indicating the level which corresponds to $50 cm$'. A $100cm$ measuring cylinder is better than one which measures $1000cm$ because $50cm$ is only a small fraction of $1000c{m^3}$.
• You want to time a toy car moving a distance of $1/0m$. You could use the clock on the wall - a poor choice. You could use a stopwatch, but it is tricky to start and stop the watch at the exact moments when the car crosses the starting and finishing lines. You would have to take account of your reaction time. It is best to use light gates since these automatically start and stop as the car passes through. The gates are connected to a time which will show you the time taken to within a fraction of a second.
Choose an accurate method of measurement.

We also need to think about how we use measuring instruments. For example:
• When using a ruler to measure the length of an object, the ruler needs to be placed directly alongside the object. Make sure that one end of the object is exactly next to the zero of the ruler's scale.

• When using a measuring cylinder, look horizontally at the surface of the liquid and read the scale level with the bottom of the meniscus.
meniscus

• When using a balance to weigh an object, check that it reads zero when there is nothing on it. Similarly, a forcemeter should read zero when no force is pulling on it. It may be possible to reset these instruments if they are not correctly set to zero.

Improving accuracy

You can see that, to make your measurements as accurate as possible, you need to think carefully about the measuring instruments you use and how you use them.
It can help to make repeat measurements; that is, to measure the same quantity several times and then to calculate the average.
With practice, you will find that your measurements become more accurate and so you will be able to trust your findings more.

Anomalous results

Paula did an experiment to find out how light intensity affects the rate of photosynthesis of a water plant. She placed a lamp at different distances from the plant, and counted the number of bubbles it gave off in one minute.
Paula made three counts for each distance of the lamp from the plant. This table shows her results.

Paula thought that one of her results didn't look right. Can you spot which one it is?
A result like this, that does not fit the pattern of all the other results, is called an anomalous result.
If you get something that looks like an anomalous result. there are two things that you can do.
1) The best thing to do is to try to measure it again.
2) If you can't do that, then you should ignore the result. So Paula should not use this result when she is calculating the mean. She should use only the other two results for that distance from the lamp, add them up and divide them by two.

Spotting an anomalous result in a results table can be quite difficult. It is often much easier if you have drawn a graph.
Ndulu did an experiment to investigate how adding ice to water changed its temperature. He added a cube of ice to 500 cm' of water and stirred the water until the ice had completely melted. Then he measured the temperature of the water before adding another ice cube. The graph on the next page shows his results.

It's easy to see that the point at $(3,3)$ doesn't fit the pattern of all the other results. Something must have gone wrong when Ndulu was making that measurement.
When Ndulu draws the line on his graph, he should ignore this result. He should also think about why it might have gone wrong. Perhaps he misread the thermometer - was the correct reading ${8^ \circ }C$? Or perhaps he forgot to stir the water and measured the temperature where the cold ice had just melted. If you think about why an anomalous result has occurred, it can help you to improve your technique and avoid such problems in the future.

Understanding equations
In Unit 10 Measuring motion you studied three equations which relate speed, distance and time. Here are the three equations:

$\begin{array}{l}speed = \frac{{distance}}{{time}}\\distance = speed \times time\\time = \frac{{distance}}{{speed}}\end{array}$

How can you remember these three equations? It will help if you think about the meaning of each quantity involved. It can also help to think about the units of each quantity:
Speed is the distance travelled per second or per hour. The word 'per' means in each', and this should remind you that the distance must be divided by the time.
Another way to think of this is to start with the units. Speed is measured in metres per second, so you must take the number of metres (the distance) and divide by the number of seconds (the time).
Distance is how far you travel. The faster you go the greater your speed), and the longer you go for the greater the time), the greater the distance travelled. This tells us that the two quantities must be multiplied together.