January 26th, 2019, 04:15 PM  #11 
Math Team Joined: Dec 2013 From: Colombia Posts: 7,600 Thanks: 2588 Math Focus: Mainly analysis and algebra  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 
Newbie Joined: Aug 2015 From: USA Posts: 12 Thanks: 0  
February 7th, 2019, 09:43 AM  #13 
Banned Camp Joined: Mar 2015 From: New Jersey Posts: 1,720 Thanks: 124 
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 
February 7th, 2019, 01:19 PM  #14  
Math Team Joined: May 2013 From: The Astral plane Posts: 2,042 Thanks: 815 Math Focus: Wibbly wobbly timeywimey stuff.  Quote:
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.  

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formulas, inverse square law, light, relativity, space, squared, things 
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