Resultant forces will cause acceleration, which can be described and calculated using Newton's laws of motion. Weight is caused by the gravitational effect of a planet attracting an object’s mass.

The weight of an object 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 is.

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)

An apple has a mass of 100 g. Calculate its weight on Earth (g = 10 N/kg).

100 g = 100 ÷ 1000 = 0.1 kg

- Question
Calculate the weight of a 30 kg dog (g = 10 N/kg).

To make a measurement of weight, we have to measure the force pulling the object towards the centre of the Earth. We do this by balancing it with a known force. If the object is stationary, Newton's third law then tells us that the known force is the same as the weight.

A spring balance will stretch until the force from the spring is enough to balance the weight of the object, and the distance the spring in it has stretched indicates the force it is exerting to hold up the object.