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Math Help - Dimensions of cylinder of a constant surface area but changing volume

  1. #1
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    Dimensions of cylinder of a constant surface area but changing volume

    Hi there! I'm new around so please feel free to correct me on forum ettiquete here!

    I am modelling a cylinder that changes volume but retains a constant surface area across a range of dimensions. With an increase in volume the cylinder decreases in length and increases in radius and vice versa for a decrease in volume.

    So far:
    S=2pi*r(r+h) - Surface area of a cylinder (eq1)
    V=pi*r2h - Volume of cylinder (eq2)

    Rearanging volume for height: h=V/(pi*r^2) (eq3)

    Substituting eq3 into eq1 yields:
    S=2pi*r(r+V/(pi*r2) = 2pi*r2+2/(V*r) = 2(V*pi*r3+1)/(V*r) (eq4)

    After failed attempts to rearrange eq4 and solve for r, I have tried a symbolic solve with matlab but the result is incredibly messy!

    Would anyone be able to offer any help as to how I may solve this problem as I expect that the method that I am trying to use is not the best for the job!

    Thanks in advance!
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  2. #2
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    Re: Dimensions of cylinder of a constant surface area but changing volume

    S = 2\pi rh + 2\pi r^2

    h = \frac{S - 2\pi r^2}{2\pi r}

    V = \pi r^2 h = \pi r^2 \cdot \frac{S - 2\pi r^2}{2\pi r} = \frac{Sr}{2} - \pi r^3
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  3. #3
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    Re: Dimensions of cylinder of a constant surface area but changing volume

    Could you explain how these equations can help me please? I need a equation solving for r in terms of V and S. Again, rearrangement produces a messy result.

    Thanks.
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  4. #4
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    Re: Dimensions of cylinder of a constant surface area but changing volume

    well ... you have the cubic equation

    V = \frac{Sr}{2} - \pi r^3

    how are you at using the cubic formula?
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  5. #5
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    Re: Dimensions of cylinder of a constant surface area but changing volume

    Thats great. Thankyou for your help! Now to implement it!
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