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January 26th, 2019, 04:15 PM   #11
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Quote:
 Originally Posted by paulm Why not the surface area of a circle or some multiple of pi?
As JeffM1 says, you are thinking too specifically.

Area of a square of side $r$: $r^2$
Area of a circle of radius $r$: $\pi r^2$
Area of a right triangle with short legs of $r$: $\frac12 r^2$
Surface area of a sphere of radius $r$: $4\pi r^2$
Surface area of a cylinder of height and radius $r$: $4\pi r^2$
Surface area of a cone of base radius $r$ and side $r$: $2\pi r^2$
Surface area of a cube of side $r$: $6r^2$

All of these have $r^2$ in common because when we measure area we always multiply two distances together. Area is proportional to the square of linear measures. Proportional means that there may be a constant term in there multiplying the linear measure, but that term doesn't change with the linear measure.

Last edited by skipjack; January 26th, 2019 at 07:52 PM. February 2nd, 2019, 04:11 AM   #12
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Quote:
 Originally Posted by v8archie Area is proportional to the square of linear measures. Proportional means that there may be a constant term in there multiplying the linear measure, but that term doesn't change with the linear measure.
Great insight, thanks. February 7th, 2019, 09:43 AM #13 Banned Camp   Joined: Mar 2015 From: New Jersey Posts: 1,720 Thanks: 126 As already mentioned, the force of gravitational and electrical attraction varies as 1/r^2. If a particle disintegrates, it radiates energy E in proportion to the mass (double the mass, double the energy). So E = kM, where k is a constant. The speed c of the energy radiated (light) is constant relative to the disintegrating particle, and E has to have units of energy. So E=mc^2 Thanks from topsquark and paulm February 7th, 2019, 01:19 PM   #14
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Quote:
 Originally Posted by zylo ...If a particle disintegrates, it radiates energy E in proportion to the mass (double the mass, double the energy)...
Sorry, I can't let these two go by. They are easy misinterpretations to make and are fairly common, so they need some attention.

What zylo is trying to say is that the total energy of a particle is given by $\displaystyle E = \sqrt{p^2c^2 + m^2 c^4}$. When we are in the reference frame where the particle is stationary, p = 0, so we get the famous $\displaystyle E = mc^2$. And the rest follows.

"Disintegrates" is also a word I'd like to draw attention to. Disintegration to my mind means "nothing left over". The better word is "decays", which implies there are some particles left over. As there is no such thing as "pure energy", decay is more the word we want to look for here.

-Dan

Last edited by skipjack; February 7th, 2019 at 05:18 PM. Tags formulas, inverse square law, light, relativity, space, squared, things Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post BenFRayfield Number Theory 0 October 15th, 2016 12:46 PM Ganesh Ujwal Physics 4 December 14th, 2014 07:25 PM moore778899 Elementary Math 0 January 16th, 2011 08:20 AM bortkiew Real Analysis 1 October 27th, 2009 10:14 AM andy Algebra 1 July 31st, 2007 12:39 PM

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