How to Estimate in IGCSE Physics

Estimation questions ask you to give the approximate value of a quantity, to the nearest order of magnitude or within a reasonable range.

Reference values to memorise

These are the typical values examiners expect:

Lengths:

• Thickness of a human hair: about 0.1 mm (1 x 10^-4 m)
• Height of a person: about 1.7 m
• Length of a car: about 4 m
• Width of a football field: about 70 m

Masses:

• Mass of a person: about 70 kg
• Mass of a car: about 1000 kg
• Mass of an apple: about 0.1 kg (100 g)

Speeds:

• Walking speed: about 1.5 m/s
• Running speed: about 8 m/s
• Speed of a car on a highway: about 30 m/s (110 km/h)
• Speed of sound in air: about 340 m/s

Times:

• Human heartbeat period: about 1 s
• Time for one school lesson: about 3600 s (1 hour)

Forces:

• Weight of a 1 kg mass: about 10 N
• Weight of a person: about 700 N

Temperatures:

• Room temperature: about 20 to 25 degrees C
• Human body temperature: about 37 degrees C
• Boiling point of water: 100 degrees C

Method for estimation questions

1. Identify what physical quantity you need to estimate.
2. Recall a reference value that is close.
3. Scale up or down as needed.
4. Give your answer to 1 significant figure or the nearest power of 10.

Example

“Estimate the kinetic energy of a person running.”

KE = 0.5 x m x v^2 m is about 70 kg, v is about 8 m/s KE = 0.5 x 70 x 64 = 2240 J Estimate: about 2000 J or 2 kJ.

The examiner would accept any value from about 1000 J to 5000 J.