[SOLVED] Simple volume density problem..

[groundknifer]

Hello, my college is some how affiliated with utexas so we gotta do all our
physics probs online and submit them to utexas... the problem is that if i
get 51.12 and the correct answer is 50.3 (tolerance 1%) then i get it
wrong!!!!!!

So heres 1 problem were i keep on getting same answer but according to
website its wrong:
http://f0rk.com/~dmitri/prob.jpg

can someone gimme correct answer please, i keep on getting 25.59 and its
wrong!!!@#!@#

THANKS!!!!!!!!!!!!!!!!!!!!!!!!!!

[enigma]

Well, I'm getting a different answer.

Can you post your work so we can see where the problem is?

[groundknifer]

Volume_top_part=1.07m x .00517m x .168m = .000929m^3
Volume_middle_part=1.07 x .00517m x .276m = .001527m^3
Volume_bottom_part=Volume_top_part

V_total=.00398m^3

density of iron is 7560 kg/m^3
density=mass/volume

so,
7560kg/m^3=mass/.00398m^3
mass=30.11

is that correct?

[groundknifer]

ok i found my mistake.. i got 25.162 (thats correct i think)

also i ran into a problem while finding the amount of iron atoms in that mass...

my equation is:
avogadros number: 6.022x10^23
atomic mass (iron): 55.85

25.162 x ((6.022x10^23)/55.85) = 2.713E+23

[enigma]

Yeah, you were counting the intersection of the T's twice. The trick is to count the top and bottom, and then use h-2d instead of h for the height of the center piece.

For the next part, the atomic mass* Avogadros number is number per gram, and you are working with kilograms.

Always keep the units in your equations. If in doubt, make sure they all cancel. That is the #A1 "gotcha" for all intro level science classes.

Welcome to Physicsforums, BTW

[Integral]

Originally posted by groundknifer
ok i found my mistake.. i got 25.162 (thats correct i think)

also i ran into a problem while finding the amount of iron atoms in that mass...

my equation is:
avogadros number: 6.022x10^23
atomic mass (iron): 55.85

25.162 x ((6.022x10^23)/55.85) = 2.713E+23

If you submit that answer my class you would still be wrong. Notice that most of the given numbers have only 3 siginificant digits. That is the most that you can have in your answer. The above number simply shows you used a calculator, it should read 25.2, given the numbers used to generate it.