Physics is a fascinating science. It deals with times that range
from less than 10-20 s (the half-life of helium 5) to 1.5 x1013 years (the probable 'age' of our
Universe). Physicists study temperatures from within a millionth of a degree above absolute zero to
almost 200 million degrees, the temperature in the plasma in a fusion reactor.
An investigation of the
mass of a quantum of FM radio radiation (2.3x10-42 kg) and the 'size' of a proton (about 1.3x10-15
m) all fall within the World of Physics!
It is vital to realise that all the quantities mentioned
above contain a number and then a unit of measurement. Without one or other the measurement
would be meaningless. Imagine saying that the world record for the long jump was 8.95 (missing out
the metres) or that the mass of an apple was kilograms (missing out the 0.30)!
All units used in
Physics are based on the International System (SI) of units which is based on the following seven base
units.
The second: this is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of caesium 137 atom. The phtograph shows a caesium atomic clock in the Science Museum, London.
The ampere: this is that constant current which, if maintained in two parallel straight conductors of infinite length and of negligible circular cross section placed 1 metre apart in a vacuum would produce a force between them of 2 x 10-7 N.
The
mole: this is the amount of substance of a system that contains as any elementary particles as there
are in 0.012 kg of carbon-12.
It is most important to realise that these units are for separate
measurements – you can't add together quantities with different units. For example five kilograms plus
twenty-five metres has no meaning. It is rather similar to having a field in which there are 30 sheep,
twelve cows and 25 pigs and asking how many there are? How many of what? It's simply a collection of
different animals - you cannot add them together!
We can use the base units to check that an equation
representing physical quantities is correct. Remember the example of the cows sheep and pigs.
Starting with ten sheep in one pen three pigs in a second pen and five cows in the third will still gives us
a mixture of sheep, pigs and cows if we let them all join in one pen.
It is the same with
equations. The 'power' of mass, length and time on one side of the equation must balance their 'power'
on the other.
This can be shown as follows. Consider the equation:
v2 =
u2 + 2as
writing down the units for each side we have:
ms-1 x
ms-1 = ms-1 x ms-1 + ms-2 x m the term m2 appears in all
terms on both sides of the equation as does the term s2, the equation balances and is therefore
probably correct. It is!
(Notice that the number two is ignored, it has no units).