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Physics Section A :
Mass and Weight Part 2 : Information Mass or Weight? In everyday-life we do not 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 kilogram(me) 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. 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. Practical Work : To Find the Relationship between Mass and Weight From the experiment we find that the slope of the graph (weight in N against mass in kg) is a straight line going through the origin, so :
The value which remains constant, in this experiment, is the pull of the Earth's gravity, and so:
Weight is dependant on gravity and so by using the above equation you can convert a mass into a weight and vice versa. As a consequence, if the gravity is ZERO the weight is ZERO :
So an object in space is weightless. We also find that the weight of a ONE kg mass is 9.81 N. We usually use 10N to make calculations easier : this means that ONE N is the weight of an average sized apple!! Gravity and therefore weight, varies slightly depending on where you are on the Earth:
In Paris and London : g = 9.81 Nkg -1 |
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