ZIMSEC O Level · Physics · Form 3
Physics depends on accurate measurement in SI units.
Base units
Length – metre (m); mass – kilogram (kg); time – second (s); current – ampere (A); temperature – kelvin (K).
Prefixes: mega (M) = 10⁶; kilo (k) = 10³; centi (c) = 10⁻²; milli (m) = 10⁻³; micro (µ) = 10⁻⁶.
Instruments
- Metre rule (to 1 mm), vernier calipers (to 0.1 mm), micrometer screw gauge (to 0.01 mm).
- Measuring cylinder: read the bottom of the meniscus at eye level.
- Stopwatch: time several swings of a pendulum and divide, to reduce error.
Scalars and vectors
Scalars have size only (mass, speed, energy, time). Vectors have size and direction (force, velocity, acceleration, displacement).
Density
Density = mass ÷ volume (ρ = m/V). Water is 1 g/cm³ = 1000 kg/m³. Objects less dense than a liquid float in it.
Worked example: A stone of mass 150 g raises water in a measuring cylinder from 40 cm³ to 100 cm³. Find its density.
Volume = 100 − 40 = 60 cm³. Density = 150 ÷ 60 = 2.5 g/cm³
Check yourself
- Convert 2.5 km to metres and 450 g to kilograms.
- Is velocity a scalar or a vector?
- Find the density of a block 2 cm × 3 cm × 5 cm with mass 81 g.
Show answers
- 2 500 m; 0.45 kg
- A vector.
- 81 ÷ 30 = 2.7 g/cm³
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