Question

if radius is doubled terminal velocity increases four times why?​

Answer 1

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Again, human 2 will have a bigger area and more air resistance. If you double the radius, the cross-sectional area will be four times as much (since area is proportional to radius squared). Now you see that the bigger human will have a greater terminal velocity.

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Answer 2

Answer:

I know that terminal velocity equals to some multiple of radius square. What is the physics behind this relationship.

Is it just because as terminal velocity increases, the surface area increases? I would say that you have the cause and effect backwards.

Explanation: