# Mass and weight

The of an object is the force acting on the object's mass due to gravity. The weight of an object can be practically measured using a newtonmeter, which is a spring balance.

The weight of an object and its mass are . This means that, for a given , doubling the mass would double the weight.

Scientists write this type of relationship in a short form as:

Weight can be calculated using the equation:

weight = mass × gravitational field strength

This is when:

• weight ( ) is measured in newtons (N)
• mass ( ) is measured in kilograms (kg)
• gravitational field strength ( ) is measured in newtons per kilogram (N/kg)

The Moon is smaller and has less mass than the Earth, so its gravitational field strength is only about one-sixth of the Earth's. For example, a 120 kg astronaut weighs 1,200 N on Earth but only 200 N on the Moon. Remember that the mass would still be 120 kg.

### Example

An apple has a mass of 100 g. Calculate its weight on Earth ( = 9.8 N/kg).

100 g = 100 ÷ 1000 = 0.1 kg

= 0.1 kg × 9.8 N/kg

= 0.98 N

Question

Calculate the weight of a 30 kg dog ( = 9.8 N/kg).

= 30 kg × 9.8 N/kg

= 294 N