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#### Edge

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I have a question on my physics home work that is kickin my butt. I have to turn it in online tonight which gives me about 8 hours to figure this one question out.<br />
<br />
Here it is:<br />
<br />
Use the fact that a typical proton is spherical and has a diameter of 1 x 10^-13 cm and a mass of 1.67 x 10^-24 grams, compute the density of a proton. A common material such as water has a density of 1 grams per cubic centimeter. Density is the mass per volume.<br />
<br />
Here are the possibles answers (multiple choice)<br />
<br />
A. 3 x 10^15 grams/cm^3 <br />
B. 2 x 10^40 grams/cm^3 <br />
C. 3 x 10^-16 grams/cm^3 <br />
D. 45 x 10^18 grams/cm^3 <br />
<br />
<br />
I know that I need to use the formula for finding the volume of a sphere "4/3 Pi r^3" but after that I am lost.<br />
<br />

#### Pythagoras

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You need to first figure out the volume of the proton using the diameter. Then you divide the mass by the volume which should give you gm/cm^3. This will give you the number that you're looking for.<br />
Something like this:<br />
<br />
4 x (1.67 x 10^-24 gm)<br />
3 x 3.1416 (pi) x (1 x 10^-13 cm)^3<br />
<br />
Or something like that!!

Edge

#### Edge

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Thanks and you are correct. I did some searching on the net and was able to figure it out before I had seen your reply. Your equation was a little easier than mine. Thank you very much for your help!<img src="/images/smilies/smile.gif" border="0" alt="" title="Smile" class="inlineimg" /> Rep for you!<br />
<br />
Here is the equation I was able to use:<br />
<br />
1*10^-13 <br />
----------= 5*10^-14 <br />
2<br />
<br />
<br />
<br />
4/3(3.14)(5^-14)^3 =5.23333333*10^-40<br />
<br />
<br />
<br />
5.23333333*10^-40<br />
--------------------=3.13373253*10^-16 Answer must be C<br />
1.67*10^-24

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