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Old 09-05-08, 05:36 PM   #1
Hickeydog
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Evil Math help

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.
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Old 09-05-08, 05:43 PM   #2
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I think the answer is ceramic ball-bearings.
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Old 09-05-08, 05:45 PM   #3
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I think the answer is ceramic ball-bearings.
That's nice. too bad it didn't answer the evil question.

Or at least tell me if you get 1.94 g/cm3
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Old 09-05-08, 05:46 PM   #4
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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.
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Old 09-05-08, 05:51 PM   #5
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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).
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Old 09-05-08, 05:53 PM   #6
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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.
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Old 09-05-08, 05:54 PM   #7
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That looks right.
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Old 09-05-08, 05:54 PM   #8
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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!
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Old 09-05-08, 05:54 PM   #9
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well

circumference = diameter x pi
volume of a sphere = 4/3 pi radius^3
density = mass/Volume
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Old 09-05-08, 05:56 PM   #10
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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.
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Old 09-05-08, 05:58 PM   #11
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never mind. I had the wrong number of sig figs.
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Old 09-05-08, 05:59 PM   #12
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the plot thickens... you sure you're entering the answer in the correct fill box?
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Old 09-05-08, 06:00 PM   #13
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Well, admittedly, if I were the teacher, I would not give you full credit because of your number of sig figs. Also, unfortunately, if all you get to do is to enter your numerical answer, you might be hosed since if your answer is close to the point where you would round up or down, then you might have to figure out/guess what approximation for pi the person used. (Using pi = 3.14 may or may not give a sufficiently large roundoff error to skew your answer slightly compared to pi = 3.14159... not enough to affect any real human grading your work, but it's not a human grading your work.)

[edit] Looks like you figured out the problem already.
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Old 09-05-08, 06:01 PM   #14
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Sig figs are the devil
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Old 09-05-08, 06:10 PM   #15
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Maybe we need to give the teacher minus points for not having them show their work.
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Old 09-05-08, 06:22 PM   #16
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never mind. I had the wrong number of sig figs.
Don't round until the end.
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