Measurements

How do measurements relate to units and physical quantities?

Units tell us the value of the physical quantity we are measuring.

The activity of measuring is done to an object to learn its properties (or physical quantity) and then compared against an accepted norm to obtain a value (the unit). See the example below:

Measuring using different unit systems

There are several different unit systems. The most universally accepted unit system is the International System of Units (SI). Other unit systems are the United States Customary System and the Imperial system.

Measurements using different systems can deliver different values, as they measure the same physical quantities using different units. An example of this is the definition of temperature for the Imperial system, which uses Fahrenheit.

Carrying out measurements

Generally speaking, there are two methods for measurements: one is by reference, while the other uses formal units.

Measuring by reference

The principle is simple: you take an object, and you use that object as a reference to measure a quantity.

Taking measurements by using formal units

This method involves using a standard so you can reproduce the measurement every time you need it.

The ability to reproduce values using an instrument as a reference is why unit systems are important in measurement.

Taking accurate measurements

Accuracy is important when taking measurements. To achieve it, you should follow these instructions:

1. Set your instruments to zero.
2. If you have instruments that use marks to read results, always read the value with your eyes directly over the values - not to one side. If you are not careful when reading the values, parallax errors might occur.Figure 2. When using instruments with marks, ensure your eyes are directly over the mark to avoid parallax errors.
3. Repeat your measurements if necessary. Many errors come from measurements that contain minor errors. You can reduce some errors by measuring several times and then averaging the results.

Representing measurements in real life

To graphically represent your measurements in real life, you can use a plot. Plots are drawings that use ‘x’ and ‘y’ variables to relate the change of one value to another. To put it simply, a plot is the graphical relationship of two or more variables. Usually, x is named as the independent variable, and y is the dependent variable.

See the example of a plot below, representing how a pendulum's movement decays over time.

Figure 3. Simple plot showing the time it takes for a pendulum to swing. The time shortens with each measurement..

The variable ‘x’ represents the measurement. The first point is the first measurement, where it takes 2 seconds for the pendulum to come and go. After the first measured value x1, you take a second measurement x2, then x3, x4 and so on.

The second point is the second measurement x2. The pendulum moves less until it slowly comes to a stop. The marks on the plot show the values decaying over time.