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Thread: Evil Math help

  1. #1
    Crushing souls Hickeydog's Avatar
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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.
    Quote Originally Posted by Wordbiker View Post

    What's frightening is how coherent Hickey was in posting that.

  2. #2
    AEO
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    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

  3. #3
    Crushing souls Hickeydog's Avatar
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    Quote Originally Posted by AEO View Post
    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
    Quote Originally Posted by Wordbiker View Post

    What's frightening is how coherent Hickey was in posting that.

  4. #4
    riding once again jschen's Avatar
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    Quote Originally Posted by Hickeydog View Post
    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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    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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  6. #6
    Crushing souls Hickeydog's Avatar
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    Quote Originally Posted by jschen View Post
    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.
    Quote Originally Posted by Wordbiker View Post

    What's frightening is how coherent Hickey was in posting that.

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    That looks right.
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  8. #8
    riding once again jschen's Avatar
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    Quote Originally Posted by Hickeydog View Post
    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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  9. #9
    AEO
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    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

  10. #10
    Crushing souls Hickeydog's Avatar
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    Quote Originally Posted by jschen View Post
    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.
    Quote Originally Posted by Wordbiker View Post

    What's frightening is how coherent Hickey was in posting that.

  11. #11
    Crushing souls Hickeydog's Avatar
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    never mind. I had the wrong number of sig figs.
    Quote Originally Posted by Wordbiker View Post

    What's frightening is how coherent Hickey was in posting that.

  12. #12
    AEO
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    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

  13. #13
    riding once again jschen's Avatar
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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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  14. #14
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    Sig figs are the devil
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  15. #15
    My tank takes chocolate. FlowerBlossom's Avatar
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    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.

  16. #16
    Senior Member biker128pedal's Avatar
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    Quote Originally Posted by Hickeydog View Post
    never mind. I had the wrong number of sig figs.
    Don't round until the end.
    Mike
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