Weight is the force acting on an object due to gravity. Close to the Earth, this force is caused by the gravitational field around the Earth. Weight is a force, so it is measured in newtons (N) and it acts downwards towards the centre of the Earth. Mass is different: it is the amount of matter in an object, it is measured in kilograms (kg) and it does not change from place to place.
The weight of an object depends on the gravitational field strength, g, at the point where the object is. On Earth g is about 10 N/kg, but on the Moon it is much smaller, so the same object weighs less there even though its mass is the same.
Weight is calculated with the equation: weight = mass × gravitational field strength, or W = m g. Weight is in newtons, mass in kilograms and g in newtons per kilogram. For example, a 3 kg object where g = 10 N/kg has a weight of 3 × 10 = 30 N, and a 4 kg object has a weight of 4 × 10 = 40 N.
The weight of an object may be considered to act at a single point called its centre of mass. Weight and mass are directly proportional, written W ∝ m, so doubling the mass doubles the weight. Weight is measured with a calibrated spring balance, also called a newtonmeter. Newtonmeters come in different ranges, such as 0 to 10 N, 0 to 50 N or 0 to 100 N.