Why chemistry/physics annoys me

[alkatran]

In physics:
x=3/2 x = 1 (significatif digits)
y=x*2 y = 2

z=3/2*2 z=3


GA! I just lost 10% on a quiz for this sort of mistake. Significatif digits are flawed anwyas... Base dependant.

Base 10:

10/4 = 2

Base 2:

1010 / 100 = 10.1 (2.5, more precision! same numbers!)

[NotLKH]

Originally posted by alkatran
??????????
10/4 = 2
??????????

I'm sorry for your circumstances, but why do you say

10/4 = 2???

Certainly Int(10/4) = 2, but useing straight forward division, you do know

10/4 = 2.5

right?

[Shaggy Hiker]

He's dealing with the number of significant digits. Since both of the operands in the equation had no digits to the right of the decimal, adding digits to the right would appear to be pseudo-precision. However, in this case, it seems that there should be two significant digits, and the result should actually be 2.5, which has two significant digits.

Been a long time since I dealt with that stuff, so I could be all screwed up.

[alkatran]

In multiplication and divition you keep the amount of SDs as the number with lowest amount of SDs.

So: 10 / 2 = (2.5)->2
(2) (1) -> (1)

In addition and subtraction you keep the least amount of SDs after the decimal.

[NotLKH]

Originally posted by alkatran
In multiplication and divition you keep the amount of SDs as the number with lowest amount of SDs.

Seems to be highly inaccurate.
Where did that rule come from?

BTW, since division by two is multiplication by 0.5, then the number of sig digits after the decimal is solely based on how you write your equation.

I would tend to argue that preservation of form trumps SD.

{lets assume you meant 5/2. since 10/2 <> 2.5}

For example,

consider 5/2 . Since this expression is expressed as a fraction, then its reduction should be of the same form.
therefore I would argue that 2 & 1/2 is a more proper expression of the reduction of the original expression than decimal.


-Lou

[bugzpodder]

yep, i gotta use that cursed rule in physics and chem too, or 0.5 marks off for each question. and in physics, if you dont right the stupid formula, you get 1 mark off for every question even if you did everything right. i lost like 5 marks on the last major test because of the stupid formulas!! i am too use to math tests where you dont need to state the formula before you go on, such as quadratic formula

[alkatran]

I guess the physics people aren't smart enough to know what you're doing, but math people are .

[siyan]

Aren't you supposed to keep everything either symbolic/with a lot of accuracy until the end, and then cut down for SD?

Heh...I avoid the whole issue now and just do symbolic manipulation until I have a answer=expression equation.

[sql_lall]

Something i have always wondered:

How many Sig. Figs is 1/3

You see, at a glance it looks like 1 (both num. & denom. are 1 sig fig) yet the number itself has infinite significant figures.

So, what would 1/3 + 0.19 be??

[alkatran]

So, what would 1/3 + 0.19 be??

Well.. I'm actually not sure when you're supposed to round off...
If it's after each erm.. level (Paranthese, ^, */, +-), or at the end.

1/3 = .3 (.3333333333333333333) .3+.19 = .5 (.49)
or
1/3+.19 = .5 (.52)

same answer here... but I should ask my teacher.

However, since my math teacher will usually keep the 1/3 instead of putting .3 I assume you only round off at the end of an equation.

[BinaryFreedom]

if i have 1 L of SO2 and i want to split it into 3 parts, how many liters would i get in each part?

1/3=0.3 i would get 0.3 L in each part. pathetic sig digs

[alkatran]

Yes, but the real phrasin for your question would be:

I have erm.. 1 litre.. I think... So if I divide it into 3 nearly equal parts.. I get...

But if you had 1.0000000 litres and divided it into 3.0000000 parts you'd get a better answer.

[A$$Bandit]

Significant figures give scientists information about how precised a measurement is, so they ARE very important. Whilst a mathematician would say that a bottle has 1/3 litres of orange juice in it, a physicist would disagree because that implies that the mathematician knows its volume to an infinite degree of precision, whereas actually he knows it's about 0.33 but his measurement could have been as much as 0.335 or as little as 0.325.

The answer to your "1/3 plus 0.19" question would be 0.33 + 0.19 = 0.52, because you have to calculate everything to the lowest number of sig figs you have.

Significant figures only apply to pure decimals because that's how they're defined, so 1/3 has no significant figures but it's decimal representation has an infinite number of them.

[alkatran]

Significant figures give scientists information about how precised a measurement is, so they ARE very important. Whilst a mathematician would say that a bottle has 1/3 litres of orange juice in it, a physicist would disagree because that implies that the mathematician knows its volume to an infinite degree of precision, whereas actually he knows it's about 0.33 but his measurement could have been as much as 0.335 or as little as 0.325.

Not that I'm disagreeing with you, but in base 3, 0.3333333333333333 (etc) = 0.1
So I could argue that we DO know it to an infinite precision
I believe every number in chemistry and physics should have a 'precision counter' next to it which would show how much it could be off by.

Remember, if you measure something to 'around' 5 cm, it (is assumed to be) between 5.5 and 4.5, so you have a loss of precision of 0.5
If you add 0.004cm to the total the loss of precision goes to 0.50005 and you have 5.004.

The argument I'd use to back this up is:

You have a ruler, you measure 2. cm then 0.5 cm, if you add them together, in physics, you will get 2. cm. To me the answer is supposed to be 2.5 with an offset of .5 (precision of 2) + .05 (.5) = .55

The reason for this is that ALL of the answer that might happen will have an average of 2.5 and will (usually) only go up to .55 away from 2.5. This means that an answer of "3" is possible... but here we are with a "2" and no clue.

Of course you would have to really suck at measuring to be a full .5 off in only two measurements...

[da_silvy]

Originally posted by alkatran
I guess the physics people aren't smart enough to know what you're doing, but math people are .

i'd say there are too many different formulas and slight differences that can change an entire equation and solution

it's easier to follow and hence to mark a problem if you're aware of the track / idea that they're on about

/end ramble

i don't know what i'm tlaking about