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Mass and Weight
Teachers Notes
This is a subject that always confuses students. It is primarily because society has come to ask the question “how much do you weigh?” and answers using the units of “kilograms” when kilograms is the SI Unit of mass which is different to weight (for completion; mass is a scalar quantity as it only has magnitude whereas weight is a vector quantity because it has both magnitude and direction. The direction component comes from the acceleration which is the vector quantity).
Weight is, actually, a force and its units are measured in the SI Unit of Newtons (N), not kilograms. It is really important to understand the difference (and if students do, there are easy marks to get in examinations).
Where additional confusion can occur is when considering mass, weight and momentum on different worlds (the Earth and the Moon being the usual examples). Remember the gravitational field strength (g) will be different on the earth (9.81 m/s ) and the moon (1.62 m/s ); therefore the weight of an object will be different on the Moon compared to the Earth (remembering F (Weight)=mg). But if you move an object horizontally on either the moon of earth, the force required to move the object (hence changing the objects momentum) is exactly the same because the horizontal force being applied is perpendicular to the direction of the weight force (ignoring friction).
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Overview Videos
This video introduces Newton’s second law Force (Newtons) = mass (Kilograms; Kg) x acceleration (metres per second per second written as m/s (also said as “metres per second, squared”)or ms ).
For GCSE, watch this through to 2 minutes, thereafter Dave is introducing KS5 Physics
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Detail Videos
The narrator describes mass beautifully. Mass is the amount of ‘stuff’ something contains and it’s the acceleration acting on that stuff that gives us weight.
Revision Videos
Freesciencelessons follows the curriculum and what students need to understand for examination purposes
This video summarises the forces acting on falling bodies where friction opposes the direction of motion (for example free fall parachutists)