Physical quantities and SI units
A physical quantity is anything we can measure. It has a numerical value and a unit. Physics is built on seven SI base units; the three met early are the metre (m) for length, the kilogram (kg) for mass and the second (s) for time. Other units, such as m/s for speed or kg/m3 for density, are derived by combining base units.
Key idea
Common prefixes: kilo (k) = 103, centi (c) = 10-2, milli (m) = 10-3, micro (µ) = 10-6. So 1 mm = 10-3 m and 1 km = 1000 m.
Choosing instruments
Match the instrument to the size measured. A ruler reads to 1 mm; vernier calipers read to 0.1 mm; a micrometer screw gauge reads to 0.01 mm for wires and thin sheets. Volumes of liquids use a measuring cylinder, read at the bottom of the meniscus with the eye level to avoid parallax error. Short time intervals use a stopwatch; for repeated events, time many cycles and divide.
Scalars and vectors
A scalar has magnitude only (distance, speed, mass, time, energy). A vector has both magnitude and direction (displacement, velocity, acceleration, force, weight).
Worked example
A student times 20 swings of a pendulum as 30.0 s. The period is T = 30.0 ÷ 20 = 1.5 s. Timing many swings and dividing reduces the effect of human reaction-time error on each measurement.
Remember
- Always quote a unit with every measured value.
- Read scales at eye level to avoid parallax error.
- Repeat readings and average to reduce random error.