1.1 Physical Quantities

All physical quantities consist of a numerical magnitude and a unit.

Estimates

• It is often necessary to estimate a quantity.
• For example, putting a large mass on a delicate, sensitive balance may damage the balance, but if we know an estimate for the mass, we can choose the appropriate balance for the measurement.
• Estimation is also helpful in knowing whether the answer to a calculation is obviously wrong.
• Since the value of an estimate cannot be known accurately, it is usually given to only one or two significant figures.

Estimations of Physical Quantities

Everyday Objects

Quantity	Estimate
Mass of an apple	100 g
Mass of plastic ruler	10 g
Mass of metre rule	100 g
Mass of protractor	10 g
Thickness of paper	0.1 mm

Human Body

Quantity	Estimate
Mass of adult	70 kg
Height of adult	2 m
Volume of head	4 000 $c{m}^3$

Constants

Quantity	Estimate
Range of audible frequency	20 Hz - 20 000 Hz
Atmospheric Pressure	101 kPa
Speed of light	300 000 000 $m{s}^{-1}$
Speed of sound in air	330 $m{s}^{-1}$

Wavelength of Visible Light

Colour	Wavelength/nm
Red	700
Orange	650
Yellow	600
Green	550
Blue	500
Indigo	450
Violet	400

Substances

Quantity	Estimate
Density of Water	1000 $kg{m}^{-3}$
Density of Air	1.3 $kg{m}^{-3}$
Density of Sand	2000 $kg{m}^{-3}$
Density of Steel	8000 $kg{m}^{-3}$
Mass of Grain of Sand	50 $\mu g$
Diameter of Grain of Sand	0.1 mm