Forces arise from the interactions of two objects. They can be divided into two categories: contact forces and non-contact forces.

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

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.

- Question
Calculate the weight of a 30 kg dog.