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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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 Originally Posted by Wordbiker What's frightening is how coherent Hickey was in posting that.

 09-05-08, 05:43 PM #2 AEO Senior Member     Join Date: Jun 2007 Location: A Coffin Called Earth. or Toronto, ON Bikes: Bianchi, Miyata, Dahon, Rossin Posts: 12,258 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) I think the answer is ceramic ball-bearings. __________________ Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;) http://sanfrancisco.ibtimes.com/arti...ger-photos.htm
09-05-08, 05:45 PM   #3
Hickeydog
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Quote:
 Originally Posted by AEO 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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 Originally Posted by Wordbiker What's frightening is how coherent Hickey was in posting that.

09-05-08, 05:46 PM   #4
jschen
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Quote:
 Originally Posted by Hickeydog 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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 09-05-08, 05:51 PM #5 Engineer101 Member   Join Date: May 2008 Location: Southwest Central, IL Bikes: late 80's Trek 400; POS MTB Posts: 44 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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).
09-05-08, 05:53 PM   #6
Hickeydog
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 Originally Posted by jschen 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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 Originally Posted by Wordbiker What's frightening is how coherent Hickey was in posting that.

 09-05-08, 05:54 PM #7 Engineer101 Member   Join Date: May 2008 Location: Southwest Central, IL Bikes: late 80's Trek 400; POS MTB Posts: 44 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) That looks right.
09-05-08, 05:54 PM   #8
jschen
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Quote:
 Originally Posted by Hickeydog 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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 09-05-08, 05:54 PM #9 AEO Senior Member     Join Date: Jun 2007 Location: A Coffin Called Earth. or Toronto, ON Bikes: Bianchi, Miyata, Dahon, Rossin Posts: 12,258 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) well circumference = diameter x pi volume of a sphere = 4/3 pi radius^3 density = mass/Volume __________________ Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;) http://sanfrancisco.ibtimes.com/arti...ger-photos.htm
09-05-08, 05:56 PM   #10
Hickeydog
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 Originally Posted by jschen 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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 Originally Posted by Wordbiker What's frightening is how coherent Hickey was in posting that.

09-05-08, 05:58 PM   #11
Hickeydog
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never mind. I had the wrong number of sig figs.
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Quote:
 Originally Posted by Wordbiker What's frightening is how coherent Hickey was in posting that.

 09-05-08, 05:59 PM #12 AEO Senior Member     Join Date: Jun 2007 Location: A Coffin Called Earth. or Toronto, ON Bikes: Bianchi, Miyata, Dahon, Rossin Posts: 12,258 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) the plot thickens... you sure you're entering the answer in the correct fill box? __________________ Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;) http://sanfrancisco.ibtimes.com/arti...ger-photos.htm
 09-05-08, 06:00 PM #13 jschen riding once again     Join Date: Oct 2005 Location: San Diego, CA Bikes: '06 Cervelo R3, '05 Specialized Allez Posts: 7,359 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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.)  Looks like you figured out the problem already. __________________ If you notice this notice then you will notice that this notice is not worth noticing.
 09-05-08, 06:01 PM #14 Engineer101 Member   Join Date: May 2008 Location: Southwest Central, IL Bikes: late 80's Trek 400; POS MTB Posts: 44 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Sig figs are the devil
 09-05-08, 06:10 PM #15 FlowerBlossom My tank takes chocolate.     Join Date: Jun 2005 Location: Olympia, WA Bikes: Trek 600 series touring bike, Trek 800 hybrid, Bianchi Posts: 6,344 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Maybe we need to give the teacher minus points for not having them show their work. __________________ Feminism is the profound notion that women are human beings.
09-05-08, 06:22 PM   #16
biker128pedal
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Quote:
 Originally Posted by Hickeydog never mind. I had the wrong number of sig figs.
Don't round until the end.

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