Units, measuring and density — GCSE Physics practice questions

Question 1

A music teacher wants to measure the diameter of a thin steel guitar string as precisely as possible.

Which instrument should she use?

Accepted Answer

B: micrometer screw gauge. A guitar string is only a fraction of a millimetre thick, so a rule or tape (resolution 1 mm) cannot measure it precisely. A micrometer screw gauge reads to 0.01 mm. A balance measures mass, not length.

Question 2

A student times a child on a playground swing.
She starts the stopwatch as the swing passes its lowest point and stops it after 12 complete oscillations.
The diagram shows the stopwatch display.

What is the period of the swing?

Accepted Answer

A: 3.12 s. The stopwatch shows 37.44 s for 12 complete oscillations. One oscillation takes 37.44 / 12 = 3.12 s. Timing many oscillations makes the reaction-time error a small fraction of the total.

Question 3

A rigid plastic bottle has an internal volume of 2.0 litres. It is full of air of density 1.2 kg / m³.
A pump forces an extra 4.8 g of air into the bottle. The volume of the bottle does not change.

What is the new density of the air in the bottle?

Accepted Answer

C: 3.6 kg / m³. V = 2.0 litres = 2.0 × 10⁻³ m³. Original mass = 1.2 × 2.0 × 10⁻³ = 2.4 × 10⁻³ kg. New mass = 2.4 × 10⁻³ + 4.8 × 10⁻³ = 7.2 × 10⁻³ kg. New density = 7.2 × 10⁻³ / 2.0 × 10⁻³ = 3.6 kg / m³.