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Calculating radius from volume

  1. Oct 21, 2009 #1
    Hi all,

    i am working through some worked examples for my course and there seems to be a step missing.

    the step says

    "the internal diameter of the vessel can be determined from"

    V=PI*r^2*4r

    Now all im given is that the volume V is 20 litres. is there any way i can determine the radius from this?

    The answer written is r=0.117m

    Thanks for any help guys!!
     
  2. jcsd
  3. Oct 21, 2009 #2
    Assuming the vessel is filled with water, do you know how many litres of water is in a cubic metre? Or how many ml makes up a cubic centimetre?
    From there you need to be able to rearrange this equation to solve for r in terms of V
     
  4. Oct 21, 2009 #3
    if I say x^3 = 8, is it ok to say x = 2
    This problem is just like the one above

    V = PI * 4 * r^3

    or r^3 = V / ( PI * 4)

    so r = ( V / (PI * 4) )^(1/3)

    Another this to remember is V is given in liters so you have to convert it into m^3.
    20 liters = .02 m^3

    and if you plug the above equation in a calculator, you get r = 0.11675m

    hope it helps
     
  5. Oct 21, 2009 #4
    Or you could just answer the entire question....
    Is it not better to give a few hints, so that the OP will understand how to do it themselves?
     
  6. Oct 21, 2009 #5
    Ahh i forgot the like terms of the r^2 and r!!!!
    i can not seem to get it to work on my calculator.
    I get for (V/Pi*4)) = 0.02/Pi*4 = 1.59*10-3
    How do i use ^(1/3)?
    Is it the abc function?
     
  7. Oct 21, 2009 #6
    ^(1/3) just means cube root, so is there a button with a 3 in a square root sign?
     
  8. Oct 21, 2009 #7
    yeah got it just. thanks mate. That was just the front end of the question. the rest is straight forward just got hung up on the rearrangment side of it. i was close but not close enough!!!!
     
  9. Oct 21, 2009 #8
    Glad to help
     
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