At what rate should the blades of the offshore turbine rotate if the speed at the tip is to be the same as that of the turbine on land?

9 rpm

Explanation:

The question is incomplete. The following is missing:

Wind turbines designed for offshore installations are much larger than ones designed for use on land. One company makes a turbine for use on land with blades 38m long that spin at 18 rpm , and a turbine for use offshore with blades that are twice as long.

The speed at the tip is to be the same means that both turbines have the same tangential speed (τ):

τ = ω*r

where ω refers to angular speed (here it will be used directly in rpm), and r refers to radius of circular path, then:

ω1*r1 = ω2*r2

Replacing with r2 = r1*2

ω1*r1 = ω2*r1*2

ω1/2 = ω2

ω2 = 18/2 = 9 rpm

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