Weight, mass and gravitational field strength

The weight of an object is defined as the force of gravity acting on the object. It may be thought of as acting at a single point called its centre of mass. Depending on the object's shape, its centre of mass can be inside or outside it.

The weight of an object and its mass are directly proportional. For a given gravitational field strength, the greater the mass of the object, the greater its weight.

Weight can be calculated using the equation: weight = mass × gravitational field strength W = m g

This is when:

  • weight (W) is measured in newtons (N)
  • mass (m) is measured in kilograms (kg)
  • gravitational field strength (g) is measured in newtons per kilogram (N/kg) On Earth, g = 10 N/kg (approximately)

Weight can be measured using a spring-balance (newtonmeter) or a top-pan balance.


An apple has a mass of 100 g. Calculate its weight on Earth.

100~g = \frac{100}{1,000} = 0.1~kg

W = m~g

W = 0.1 \times 10

W = 1.0~N


Calculate the weight of a 30 kg dog.

W = m~g

W = 30 \times 10

W = 300~N

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