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Old Apr 29th 2017, 07:37 AM   #1
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measurement

why we use unit with magnitude while measuring physical quantity,
can t' we just measure magnitude,
in physics due to this units we need to remember lots of formulas.
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Old Apr 29th 2017, 07:53 AM   #2
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Would you like to give an example?
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Old Apr 29th 2017, 08:30 PM   #3
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speed of light=3*10 power -8
charge of electron =1.6*10 power -11

length=12
mass=43
why we use units like kg,m,cm,g so much can t ' we just write magnitude physical quantity.
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Old Apr 30th 2017, 02:39 AM   #4
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I am still not understanding you fully so here is my example.

You might think the radius of the Earth is 6400

but I would say it was 4000

and we would both be right because you are measuring in kilometres and I am measuring in miles.

Does this help?
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Old Apr 30th 2017, 05:09 AM   #5
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yes exactly right
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Old Apr 30th 2017, 05:31 AM   #6
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Standardised versions of these units can also help us in physics.

We can derive pretty well all of classical physics from a few basic units, called dimensions.

Mass - M
Length - L
Time -T
Temperature - $\displaystyle \theta $(greek theta)
Current - I

This can for instance tell us if we have an equation correct or compare quantities

so kinetic energy = $\displaystyle \frac{1}{2}m{v^2}$


$\displaystyle KE = \frac{1}{2}m{v^2} = \frac{1}{2}M{\left( {\frac{{dis\tan ce}}{{Time}}} \right)^2} = M{L^2}{T^{ - 2}}$

and potential energy = mgh or


$\displaystyle PE = mgh = M\left( {\frac{{velocity}}{{Time}}} \right)L = M\frac{{\frac{{dis\tan ce}}{{time}}}}{{time}}L = M{L^2}{T^{ - 2}}$


and we can see that both have the same dimensions (so long as we work in the same units) - and these are the dimensions of energy.

Note we do not include the dimemsions of the half in the kinetic energy because it is just a number, but some constant have dimensions, as with the aceleration due to gravity in the potential energy formula and these must be included.
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Last edited by studiot; Apr 30th 2017 at 12:42 PM.
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