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Science

Forces

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Weight, mass and gravity

People often confuse mass and weight. Remember that weight is a force, and is measured in newtons. Mass is measured in kilograms (kg).

Mass

The mass of an object is the amount of matter or "stuff" it contains. The more matter an object contains, the greater its mass. An elephant contains more matter than a mouse, so it has a greater mass. Mass is measured in kilograms, kg, or grams, g.

A 100 kg object has a greater mass than a 5 kg object. Remember an object's mass stays the same wherever it is.

Gravity

All objects have a force that attracts them towards each other. This is called gravity. Even you attract other objects to you because of gravity, but you have too little mass for the force to be very strong.

Gravitational force increases when:

  • the masses are bigger

  • the objects are closer

Gravity only becomes noticeable when there is a really massive object like a moon, planet or star. We are pulled down towards the ground because of gravity. The gravitational force pulls in the direction towards the centre of the Earth.

Shows how gravity keeps us on the Earth

"Down" is towards the centre of the Earth, wherever you are on the planet

Weight

Weight is a force caused by gravity. The weight of an object is the gravitational force between the object and the Earth. The more mass the object has the greater its weight will be.

Weight is a force, so it's measured in newtons. On the surface of the Earth an object with a mass of 1 kg has a weight of about 10 N.

Mass and weight

Remember that mass is measured in kilograms, kg, and weight is measured in newtons, N.

The mass of an object stays the same wherever it is, but its weight can change. This happens if the object goes somewhere where gravity is stronger, or weaker, such as the Moon.

The Moon has less mass than the Earth, so its gravity is less than the Earth's gravity. This means that objects weigh less on the Moon than they do on the Earth.

The Moon's gravity is one sixth of the Earth's gravity. A 120 kg astronaut weighs 1200 N on Earth. On the Moon they would weigh only 200 N. The astronaut's mass is 120kg wherever they are.

On Earth, an astronaut has a mass of 120kg. His weight is 120 x 10 = 1200N. On the moon his mass is 120Kg but his weight is 200N

The weight of an object changes if the strength of gravity changes

Remember: a mass of 1 kg has a weight of about 10 N on Earth.

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