Physics Help Forum measurement

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 Apr 29th 2017, 07:37 AM #1 Banned   Join Date: Nov 2016 Posts: 13 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.
 Apr 29th 2017, 07:53 AM #2 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 815 Would you like to give an example?
 Apr 29th 2017, 08:30 PM #3 Banned   Join Date: Nov 2016 Posts: 13 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.
 Apr 30th 2017, 02:39 AM #4 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 815 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?
 Apr 30th 2017, 05:09 AM #5 Banned   Join Date: Nov 2016 Posts: 13 yes exactly right
 Apr 30th 2017, 05:31 AM #6 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 815 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. kavinmathi likes this. Last edited by studiot; Apr 30th 2017 at 12:42 PM.

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