Energy is a key principle in physics. Forces cause changes and energy calculations allow us to measure how much change can happen.

The weight 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 directly proportional. This means that, for a given gravitational field strength, 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.

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