# Foo - Evil Math help

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View Full Version : Evil Math help

Hickeydog
09-05-08, 06:36 PM
HALP!!!! I have a problem. A problem that goes like this:A steel ball-bearing with a circumference of 32.5 mm weighs 4.20 g. What is the density of the steel in g/cm3 (V of a sphere = 4/3 πr3; circumference of a circle = 2πr)?

I keep trying it and I get 7.245g/cm3 (that is grams per centimeter cubed). I need help. Plz. Your math expertise is in dire need.

AEO
09-05-08, 06:43 PM
I think the answer is ceramic ball-bearings.

Hickeydog
09-05-08, 06:45 PM
I think the answer is ceramic ball-bearings.

Or at least tell me if you get 1.94 g/cm3

jschen
09-05-08, 06:46 PM
HALP!!!! I have a problem. A problem that goes like this:A steel ball-bearing with a circumference of 32.5 mm weighs 4.20 g. What is the density of the steel in g/cm3 (V of a sphere = 4/3 ?r3; circumference of a circle = 2?r)?

I keep trying it and I get 7.245g/cm3 (that is grams per centimeter cubed). I need help. Plz. Your math expertise is in dire need.

Where exactly are you running into problems? What makes you think that you're doing something wrong? Show your work if you're confused.

Engineer101
09-05-08, 06:51 PM
Yeah, I just did the problem, and I got the same answer. Plus, I looked up the density of steel and it is about 7.85 g/cm^3, so it's not that far off, especially for a random problem (assuming that's what it is).

Hickeydog
09-05-08, 06:53 PM
Where exactly are you running into problems? What makes you think that you're doing something wrong? Show your work if you're confused.

Okay. So I start out with a ball with a circumference of 32.5mm. Am I correct in thinking that this equals 3.25cm? I then divide that by 2pi to get the radius, which I get to be 0.517253cm. Then I insert it into the sphere equation (4/3 times pi times the radius cubed) which is 0.579692 cubic centimeters. Then I take 4.2g and divide that by 0.579692 cubic centimeters to get density in grams per cubic centimeter.

Engineer101
09-05-08, 06:54 PM
That looks right.

jschen
09-05-08, 06:54 PM
Okay. So I start out with a ball with a circumference of 32.5mm. Am I correct in thinking that this equals 3.25cm? I then divide that by 2pi to get the radius, which I get to be 0.517253cm. Then I insert it into the sphere equation (4/3 times pi times the radius cubed) which is 0.579692 cubic centimeters. Then I take 4.2g and divide that by 0.579692 cubic centimeters to get density in grams per cubic centimeter.

So where's the problem? Have some faith in yourself!

AEO
09-05-08, 06:54 PM
well

circumference = diameter x pi
volume of a sphere = 4/3 pi radius^3
density = mass/Volume

Hickeydog
09-05-08, 06:56 PM
So where's the problem? Have some faith in yourself!

The problem is that the correct answer is not that. I'm doing homework online where you get 3 chances to make the correct answer. If you enter one inccorect, it tells you that it is incorrect.

Hickeydog
09-05-08, 06:58 PM
never mind. I had the wrong number of sig figs.

AEO
09-05-08, 06:59 PM
the plot thickens... you sure you're entering the answer in the correct fill box?

jschen
09-05-08, 07:00 PM

 Looks like you figured out the problem already.

Engineer101
09-05-08, 07:01 PM
Sig figs are the devil :bang:

FlowerBlossom
09-05-08, 07:10 PM
Maybe we need to give the teacher minus points for not having them show their work.

biker128pedal
09-05-08, 07:22 PM
never mind. I had the wrong number of sig figs.

Don't round until the end.