L2 - A System of Units

►There are many different units - including those above. But in scientific work, life is much easier if everyone uses a common system of units.

SI units

►Most scientists use SI units (full name: Le Systeme International d’Unites). The basic SI units for measuring mass, time, and length are the kilogram, the second, and the metre. From these base units come a whole range of units for measuring volume, speed, force, energy, and other quantities.

►Other SI base units include the ampere (for measuring electric current) and the kelvin (for measuring temperature).

Mass

►Mass is a measure of the amount of substance in an object. It has two effects:

§All objects are attracted to the Earth. The greater the mass of an object, the stronger is the Earth’s gravitational pull on it.

§All objects resist attempts to make them go faster, slower, or in a different direction. The greater the mass, the greater is the resistance to change in motion.

►The SI base unit of mass is the kilogram (symbol kg). The standard kilogram is a block of platinum alloy kept at the Office of Weights and Measures in Paris. Other units based on the kilogram are shown below:

►The SI base unit of time is the second (symbol s). Here are some shorter units based on the second:

1 millisecond (ms) = 1/(1 000) s = 10^-1s

1 microsecond (μs) = 1/(1 000 000) s = 10^-6s

1 nanosecond (ns) = 1/(1000 000 000) s = 10^-9s

►To keep time, clocks and watches need something that beats at a steady rate. Some old clocks used the swings of a pendulum. Modern digital watches count the vibrations made by a tiny quartz crystal.

Length

►There are larger and smaller units of length based on the metre: