Mass and weight

In physics, mass and weight are two different concepts. Mass is the measure of the amount of matter, or 'stuff', an object is made of. It's a fundamental property of an object and is measured in kilograms (kg). Mass is also a measure of inertia – how resistant an object is to changing its motion. Weight, on the other hand, is a force. It is the force of gravity pulling on an object's mass. Because it's a force, weight is measured in newtons (N). An object's mass is the same everywhere in the universe, but its weight can change depending on the strength of the gravity it is experiencing.

Any object with mass creates a gravitational field around it. This field is a region of space where another mass will experience a gravitational force. The strength of this field at any point is called the gravitational field strength, given the symbol 'g'. It is defined as the force per unit mass. On the surface of the Earth, the gravitational field strength is approximately 10 newtons per kilogram (10 N/kg). This means that for every 1 kg of mass, the Earth pulls on it with a force of 10 N. This value is different for other planets and moons; for example, on the Moon, g is only about 1.6 N/kg.

The relationship between weight, mass, and gravity is summarised by a simple but crucial equation: Weight = mass × gravitational field strength. In symbols, this is written as W = mg. To use this formula correctly, you must use SI units: Weight (W) in newtons (N), mass(m) in kilograms (kg), and gravitational field strength(g) in newtons per kilogram (N/kg). For all IGCSE exam questions, unless told otherwise, you should use the value g = 10 N/kg for Earth. The symbol 'g' also represents the 'acceleration of free fall'. An object falling freely under gravity (with no air resistance) on Earth will accelerate at 10 metres per second squared (10 m/s²). The units N/kg and m/s² are equivalent.

Since mass and weight are different quantities, they are measured with different instruments. Mass is measured using a balance, such as a traditional beam balance or a modern electronic top-pan balance. A beam balance works by comparing the weight of an unknown object with the weight of known standard masses. Because gravity pulls equally on both sides, if the beam is level, the masses must be equal. This means a beam balance will give the same reading on the Earth or the Moon. Weight is measured using a calibrated spring balance, also called a newton meter. This device measures force by the amount a spring stretches. Since weight is a force, a newton meter directly measures it. An object's weight reading on a newton meter would be much less on the Moon than on Earth.