Measuring Length & Time
Measuring Length
SI unit: metre (m).
- Measuring tape — for curved lengths. Smallest reading: about 1 mm.
- Metre rule — for linear lengths. Smallest reading: typically 0.1–1.0 cm (depends on graduations).
- Micrometer screw gauge — for very small thickness. Least count: 0.01 mm.
Using a Rule (avoid parallax error)
- Place the scale right next to the object; align one end with zero (or note exact start mark).
- Place eye perpendicular to the mark at the other end.
- If you didn’t start at zero, compute final − initial.
Examples
- Offset start: Start 1.0 cm, end 3.7 cm → length = 2.7 cm.
- Rolling cylinder (2 turns): Start 2 cm, end 28 cm → total = 26 cm for 2 turns → circumference = 13 cm.
Micrometer Screw Gauge
- Place the object between the anvil and spindle.
- Tighten with the ratchet for consistent contact.
- Main (sleeve) scale: read mm (and half-mm), e.g. 2.5 mm.
- Thimble scale: divisions × 0.01 mm, e.g. 46 → 0.46 mm.
- Total = 2.5 + 0.46 = 2.96 mm.
Precaution
For sheets/wires, take several readings at different points and use the average.
Measuring Time
Instrument: Stopwatch. SI unit: second (s).
Pendulum Method (time period)
- Displace the bob by a small angle (≈ ≤15°).
- Start timing as it passes the reference point; count 20 oscillations.
- Stop timing; compute average period
T = \u0074\u006F\u0074\u0061\u006C \u0074\u0069\u006D\u0065 / 20.
Useful Relations
T = 2π √(L / g) (simple pendulum) a = Δv / Δt
Accuracy & Good Practice
- Begin/end timing exactly at the reference point.
- Repeat trials and take the average.
- Quote values with appropriate significant figures; include units.
- Check for zero error and avoid parallax.