La problema de matimatica


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YoPedro
January 11, 2007, 01:18 PM
Okay, so I have heard that as we get older, we get smarter. How many of you have used the cliche', "If I only knew back then what I know now." Well, if that's the case, how come I can't remember how to do simple percentages and other basic math?

My problem is grain calculation as a percentage. If for example I am adjusting a load (.45 ACP) from 5.2g to 5.3g that is an increase of what %? How about the reverse? On the other hand is the adjustment of 3.8g to 3.9g, (%?). The smaller numbers show a greater increase as a percentage relative to each load, but I can't recall the formula for a percentage soluttion.

If I want to adjust a load by 5% or 10% is that the right approach or do you simply move the grains and damn the percentages? My range notes get discombobulated because I note that I want to make an adjustment of 10% (+/-) but I outsmart myself because when I get back to the reloading bench, I can't do the math.

My kids are no help, they're in public school! Just kidding, they're too young. Maybe if I beat them more often they'll get smarter. Nah, it didn't work for me.

I figure just about anybody that remembers to put socks on has got to be smarter than me, so if anyone out there in bang bang land has the formula or a math lesson, I sure would be appreciative.

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Otherguy Overby
January 11, 2007, 01:46 PM
Wanna add 10 percent? Just multiply your starting load by 1.1

For 5%, multiply by 1.05

Added:

Of course you should also think about the result. IOW, does it make sense. If you add 10% to a 5.2 grain load, that's .52 grains, is that a larger amount that might not be wise?

If you add 10% to a load, the pressure can go up much more than 10%

Bula
January 11, 2007, 02:15 PM
to figure percentages from one grain to another (ie. 5.2 gr to 6.2 gr is what %)

6.2 minus 5.5=.7
.7 divided by 5.5 (the smaller of the two above #'s)=.127 or 12.7%

So an increase TO 6.2 FROM 5.5 represents an increase of 12.7%

To double check yourself.
Multiply the 12.7% by the lesser weight 5.5:
.127 Mult by 5.5=.6985 (rounded to the original .7 difference in the two above charges)


To bump a % up, say, 15%:

convert the desired increase to a decimal (move decimal 2 spaces to the left) ie 15% becomes .15 and 5% becomes .05

Multiply your beginning charge of 5 gr. by .15, this total is .75, then add that .75 back to your original charge of 5 gr
So 5 gr bumped up 15% would be 5 gr + .75 gr= 5.75 gr.

To lessen a charge you subtract your figure from the original charge. ie a 15% lesser charge from 5 gr would be 5 minus .75= 4.25 gr.

Bula
January 11, 2007, 02:30 PM
Yo Pedro, I'd be carefull developing loads using percentages instead of actual weight in grains or tenths of a grain.

Phantom Warrior
January 11, 2007, 03:00 PM
HA! A math question. I knew my math degree would be useful for SOMETHING.


Ok, you've asked two questions.

Number One: PERCENT OF CHANGE

If you are trying to figure out what percentage a change in grains is (5.2 grains to 5.3 grains, for example) that is a PERCENT OF CHANGE. The formulas is as follows:

PERCENT OF CHANGE = (New amount - old amount) / old amount

Using the example: 5.2 grains to 5.3 grains

Percent of change = (5.3 - 5.2) / 5.3 = 0.1 / 5.3 = 0.0189 = ~0.02
Roughly a 2% increase.


In reverse: 5.3 grains down to 5.2 grains

Percent of change = (5.2 -5.3) / 5.2 = -0.1 / 5.2 = -0.0192 = ~ -.02
The negative just means it's a decrease this time (obviously), so we see roughly a 2% decrease.


Just make sure to keep track of which is the new amount and which is the new amount. Especially when you divide the difference by the OLD amount. If you use the wrong number it will, of course, throw off your answer.



Number two: PERCENTAGES
Ok, if you want to change the powder weight by 5% or 10% you are dealing with a PERCENTAGE.

PERCENTAGE = (1 +/- the percent of change) * original weight


An increase: 5.2 grains, plus 10%

10% as a decimal is 0.10 (just move the decimal point two places left).

Percentage = (1 + [plus because it's an INCREASE] 0.10) * 5.2 = (1.10) * 5.2 = 5.72


A decrease: 5.2 grains, minus 10%

Once again, 10% is 0.10.

Percentage = (1 - [minus since it's a decrease] 0.10) * 5.2 = (0.90) * 5.2 = 4.68



Hopefully that makes some sense. Feel free to ask for clarification is I did a bad job of explaining it.

YoPedro
January 11, 2007, 08:24 PM
I can't thank you all enough for the refresher course in math. Naturally, I change my loads in 1/10th of a grain at a time, but what I wanted to resolve was the mass differential between larger loads versus smaller, and the performance curve therein. A one tenth grain move from 3.8g to 3.9g is far more consequntial then say between 5.2g and 5.3g.

However, there are other variables that determine results when calculating these factors. So, if at the range I want to alter the load by a percentage, I can use these formulas to determine the closest grain weight. If I want to eek out another 25fps on the chronograph, I would like to be able to know that an increase or decrease of overall mass via a ratio relative to the base line could or will give me the results I hope to achieve.

It seems to me that rather than just loading up and banging away with different loads a well thought out calculated system would yield far more immediate and satisfying results.

Now i r smort nuf to dose it.

LHB1
January 11, 2007, 09:42 PM
PERCENT OF CHANGE = (New amount - old amount) / old amount

Using the example: 5.2 grains to 5.3 grains

Percent of change = (5.3 - 5.2) / 5.3 = 0.1 / 5.3 = 0.0189 = ~0.02
Roughly a 2% increase.


In reverse: 5.3 grains down to 5.2 grains

Percent of change = (5.2 - 5.3) / 5.2 = -0.1 / 5.2 = -0.0192 = ~ -.02
The negative just means it's a decrease this time (obviously), so we see roughly a 2% decrease.


Just make sure to keep track of which is the new amount and which is the new amount. Especially when you divide the difference by the OLD amount. If you use the wrong number it will, of course, throw off your answer.

Phantom Warrior,
Your words are correct but you made the same mistake you warned him about. You divided by the NEW weight instead of the OLD weight.

Examples should be:

Using the example: 5.2 grains to 5.3 grains

Percent of change = [(5.3 - 5.2) / 5.2] x 100 = (0.1 / 5.2) x 100 = 0.0192 x 100 = 1.9%

In reverse: 5.3 grains down to 5.2 grains

Percent of change = [(5.2 -5.3) / 5.3] = (- 0.1 / 5.3) x 100 = - 0.0189 x 100 = - 1.9%

Good shooting and be safe.
LB

Phantom Warrior
January 17, 2007, 01:44 PM
:eek:

I am stricken with remorse. LHB1 is correct. I guess that just goes to show the importance of CHECKING YOUR NUMBERS!

The Bushmaster
January 17, 2007, 02:14 PM
Thank you, Phantom Warrior, for the service and sacrifice you are giving our country. Hurry home...Safely...:)

Eagle103
January 17, 2007, 03:56 PM
Your words are correct but you made the same mistake you warned him about. You divided by the NEW weight instead of the OLD weight.

I actually noticed that too.:eek: I think I went to school for far too long.

Phantom Warrrior, hope all is well over there. I live not far from you over here, just west of Park Rapids. Thank you for your service sir.

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