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Volumes and heights of cones.

  1. May 12, 2016 #1
    1. The problem statement, all variables and given/known data

    upload_2016-5-12_18-58-44.png
    2. Relevant equations
    volume of cone = pi(r)2h / 3
    volume of cylinder = pi(r)2h

    3. The attempt at a solution
    How can I do this, all they give is the volume and we know the value of pi ( 3.14159). I tried but you can't do it without knowing at least the radius or the height.
     
  2. jcsd
  3. May 12, 2016 #2
    That is the exact wording there is nothing else to it, i might have not understood or it just doesn't make sense..
     
  4. May 12, 2016 #3
    By just looking at the equation you have written, What do you need to do the cylinder volume in order to get the cone volume ?
     
  5. May 12, 2016 #4
    the height and radius is the same of both objects
     
  6. May 12, 2016 #5
    yea they both have the same height and radius but i still can't find out the height or radius with just the volume.
     
  7. May 12, 2016 #6
    look again at the equations you have written, and only at them. what is the same and what is different, type it here.
     
  8. May 12, 2016 #7
    volume of cone = pi(r)2h / 3
    volume of cylinder = pi(r)2h

    They both use pi and the radius is squared, only different is for a cone you have to divide by 3. I'm gonna take a crack at it but it might be wrong.
    So to find the radius we would do as shown below :

    120 x 3 (we are reversing the original formula)

    360/ 3.14 = 114.64

    114.64 squared are equal to 10.707.

    So the radius of both
     
  9. May 12, 2016 #8
    Why did you do that?
     
  10. May 12, 2016 #9
    don't you multiply 3.14 by 360, the reverse of that would be division.
     
  11. May 12, 2016 #10
    no need to. we need to find the volume of the cylinder.

    What happens if you multiply by 3 your first equation. don't use the 120 yet. just the first equation.
     
  12. May 12, 2016 #11
    Sorry just so im clear what are you refering to when you say "first equations". Do you mean the volume of the cone or
     
  13. May 12, 2016 #12
    this is the first equation you have written pi(r2)2h / 3 = Volume of cone
     
  14. May 12, 2016 #13
    no numbers yet, just use the parameters, manually multiply it by 3 and type what you got
     
  15. May 12, 2016 #14
    Ok just making sure so i don't mess up. If i multiply it by 3 it would become 3 Pi(h)r2??
     
  16. May 12, 2016 #15
    wasn't there a /3 aswell? where did it go?
     
  17. May 12, 2016 #16
    wait doesn't the multiplied 3 cancel out the other division one?
     
  18. May 12, 2016 #17
    Type it all down from the start, don't show only the result , and let's see
     
  19. May 12, 2016 #18
    wait that only works if it was a division 3 that cancels a division 3.
     
  20. May 12, 2016 #19
    1/3 * 3 = ? any division by three can be written as x/3 = x* (1/3)
     
  21. May 12, 2016 #20
    it would = 1
     
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