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**florx** The thrust, T, delivered by a ship's propeller is proportional to the square of the propeller rotation speed, R, times the fourth power of the propeller diameter, D.

a) What happens to the thrust if the propeller speed is doubled?

b) What happens to the thrust if the propeller diameter is doubled?

c) If the propeller diameter is increased by 50%, by how much can the propeller speed be reduced to deliver the same thrust?

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I have found out the equation to this to be "T = k*R^2*D^4.

What I don't understand is how to answer the above questions. If it is proportional then wouldn't the answer to a) be increase by a factor of 4? And the answer to b) be increased by a factor of 8? Also it would be greatly appreciated if you could help me with c).