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 Weight   On Earth, the weight of an object depends on the gravitational pull of the Earth. This gravitational pull is a force (see section on force) which acts towards the centre of the Earth. Weight is measured using a newton-meter (spring balance or dynamometer) and it is measured in the SI unit the newton (N). Since weight is dependant on gravity, it varies if gravity varies. For example, the gravity on the Moon is one sixth of the Earth's gravity.

PHYSICAL SCIENCE

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Mechanics

Mass and Weight

In everyday-life we do not always make a distinction between the mass and weight of an object. However, in science, we must make this distinction because mass and weight are not the same.

Mass

This is the measure of the amount of matter (solid, liquid or gas), in a substance.

Mass is measured using a balance and is measured in the SI unit the kilogramme (kg). Since the mass depends on the the amount of matter in an object, it is a constant.

Therefore, mass remains the same wherever it is on the Earth, on the Moon or on Mars.

The Moon's Mass

This is responsible for the gravitational pull of the Moon.

The Moon is approx. ¼ of the Earth's diameter and so we could assume that its pull of gravity should be approx. ¼ of the Earth's. However, from experiments performed on the Moon's surface, we know that its centre is not made of as much nickel and iron as the Earth's : as a consequence, the pull of the centre of the Moon is 1/6 of the Earth's gravity. Imagine the Olympics taking place on the Moon!

The Moon's Gravity

Galileo performed an experiment from the top of the Leaning Tower of Pisa ; where two balls of different masses were dropped. These two balls hit the ground at the same time. Neil Armstrong repeated Galileo's experiment on the Moon's surface. He dropped a hammer and a large feather, an ostrich feather, I think. Since there is no atmosphere and therefore no air resistance; the feather and hammer hit the Moon's surface at the same time!

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