Forces are responsible for interactions between objects; gravity being one of the most important. An object with mass in a gravitational field experiences a force known as weight.

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All objects with mass produce a gravitational field around them. The more mass an object has, the greater its gravitational field will be.

Gravitational field strength (g) is measured in newtons per kilogram (N/kg). The Earth's gravitational field strength at, or close to, the surface is 10 N/kg. This means that for each kilogram of mass, an object will experience 10 N of force.

Where there is a weaker gravitational field, the weight of an object is smaller. For example, the gravitational field strength of the Moon is 1.6 N/kg. This means that for each kilogram of mass, an object will experience 1.6 N of force. Therefore, an astronaut will weigh less on the Moon than they do on the Earth.

Weight is the force acting on an object due to gravity - it has the unit newtons (N) and acts towards the centre of a gravitational field. The weight of an object can be measured using a spring-balance often called a force meter calibrated in newtons and sometimes called a newton meter.

Weight is a non-contact force because gravity exerts its force through a field. An object does not need to be touching the Earth to have a weight.

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)

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

100 g = 100 ÷ 1,000 = 0.1 kg

- Question
Calculate the weight of a 30 kg dog (

*g*= 10 N/kg).