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Teacher
Notes |
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This page is contains a number of notes for teachers. Over time, additional
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General
Notes |
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- Weight versus Mass. Although the kilogram is commonly used as a measure of
weight, it is
actually defined as a measure of mass. Weight is a measure of how heavy
something is, whereas mass is a measure of the amount of matter. To illustrate,
a 120 pound woman would weigh only 20 pounds on the moon (due to the moon having
lower gravity than the earth), whereas a 50kg woman is 50kg on both the earth
and the moon. In other words, her weight (how heavy she is) is less on the moon
but her mass (how much matter she has) is unchanged. For everyday use, the
difference between mass and weight is irrelevant, as everyone is together on the
earth. However, for scientific calculations (e.g. objects being launched into
space), the difference is important.
- Celsius versus Kelvin. In the SI, temperature is officially
measured in degrees Kelvin, but in day-to-day use the Celsius system is used
instead. The original metric measure of temperature was the Celsius system,
where 0°C was set to the freezing point of water. Officially,
this has now been replaced by the Kelvin system, where 0°K
is set to absolute zero (e.g. the temperature of outer space, at a point
completely remote from any source of heat). However, it is
much more natural to say 'what a hot day, it must be 40°C' than to say 'what
a hot day, it must be 313°K'. Consequently, the system Celsius lives on in
common usage.
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