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*From*: chuck britton <cvbritton@mac.com>*Date*: Mon, 06 Feb 2012 03:19:47 -0500

Proofs? Do algebra based courses require proofs? Fifth grade courses certainly don't.

Any course in science that uses percentages should include exponential growth/decay and should include the rough approximation that relates growth rate (% / time) and half-life.

I've heard bankers call this the 'Rule of 72' since 72 has many factors that used to be equal to 'reasonable' % interest rates.

This 'rule' is only good for small percentages but has enough usefulness to be taught and used much more than it is currently.

.

At 1:00 PM -0800 2/5/12, Bernard Cleyet wrote:

On 2012, Feb 05, , at 12:26, John Denker wrote:

3) More generally: For any X that is an exponential function of t,

calculate É¢X/É¢t for any fixed finite É¢t ... not necessarily equal to

the half-life. No need to take the limit dX/dt; finite differences

work just fine. Observe that the rate of change in X is proportional

to X.

OK, this works, but just the graph doesn't, as unless the range is sufficient, a power law or even quartic, etc. looks like an exponential. I just used the model function in Graphical analysis to check this.

Of course, once one has plotted, say growth, one may use GA to fit various functions to discover it is indeed exponential.

For decay. I before using a short (~ one minute) half life source give the exercise of one thousand dice removing the ones after each throw. This also shows the scatter, especially if ten groups throwing initially 100 and then successively combining their results. One may apply, approximately, the Poisson distrib. to this too.

Not incidentally, graphing is not proof? Is the differential eq. proof?

bc wonders if graphing is derivation.

**References**:**[Phys-l] block sliding on a skateboard***From:*"LaMontagne, Bob" <RLAMONT@providence.edu>

**Re: [Phys-l] block sliding on a skateboard***From:*John Denker <jsd@av8n.com>

**Re: [Phys-l] block sliding on a skateboard***From:*Bernard Cleyet <bernardcleyet@redshift.com>

**[Phys-l] scope of the course***From:*John Denker <jsd@av8n.com>

**Re: [Phys-l] scope of the course***From:*Bernard Cleyet <bernardcleyet@redshift.com>

**Re: [Phys-l] scope of the course***From:*John Denker <jsd@av8n.com>

**Re: [Phys-l] scope of the course***From:*Bernard Cleyet <bernardcleyet@redshift.com>

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