Mass, weight and gravitational field strength
This is Cambridge International IGCSE Physics 0625, separate from the Egyptian Baccalaureate / Thanaweya Amma and not a Year 3 course. Learn why mass and weight are different, use W = mg, and practise how a balance compares masses. Supplement / Extended field ideas are labelled.
Mass tells us how much matter there is
Mass describes the amount of matter in an object. A full bag and an empty bag have different masses because they contain different amounts of stuff. Mass is measured in kilograms (kg) or grams (g). Moving the same object from Earth to the Moon does not change its mass.
Open the worked example
A 2 kg suitcase has a mass of 2 kg in Cairo, on a mountain, or on the Moon. It has not gained or lost any of its matter just by travelling.
Quick check: An astronaut takes a 70 kg equipment case to the Moon. What is the case’s mass there?
Think, then reveal: 70 kg. Mass measures the object’s matter, so a change of location does not change it.
Explore mass, weight and balance
Interactive model · same mass, different weight
Change the mass and place. The matter stays the same; the gravitational field changes the force.
W = mg = 10 kg × 9.8 N/kg = 98.0 N
On Earth, this object’s mass is still 10 kg; its weight is 98.0 N.
Same mass: 10 kg · g = 9.8 N/kg
Same mass: 10 kg · g = 1.6 N/kg
Same mass: 10 kg · g = 3.7 N/kg
Try a beam balance
A known 5.0 kg mass sits on the right pan. Adjust the unknown object and compare on Earth or the Moon.
Object: 5.0 kg × 9.8 N/kg = 49.0 N · Standards: 5.0 kg × 9.8 N/kg = 49.0 N
A weaker field reduces both weights by the same factor. It does not change which mass is larger, so the balance still compares mass.
Practise, then read the explanation
Questions marked Supplement / Extended include the gravitational-field explanation. Try first, open the optional hint if helpful, then submit to see the reasoning.
Optional hint
Mass describes the object itself, not the strength of gravity around it.
Original lesson and practice align to Cambridge IGCSE Physics 0625 syllabus links 1.3.1–1.3.5 as grouped in the supplied Volume A. Mass, weight, gravitational field strength and the balance comparison are taught as Core. The explanation of weight as the effect of a gravitational field on mass is marked Supplement / Extended. Source reading: Physics-0625 Volume A, PDF pp. 43–48 (printed pp. 35–40). The interactive values use the stated example field strengths; they are not measurements.