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Chapter 7 - 7.37

37. We write the force as F = αv ,where v is the speed and α is a constant of proportionality. The power

required is P = Fv = αv 2 .Let P 1 be the power required for speed v 1 and P 2 be the power required for

speed v 2 . Dividing P 2 = αv 2 by P 1 = αv 1 , we ﬁnd

P 2 = v 2

v 1

P 1 .

Since P 1 =7 . 5kWand v 2 =3 v 1 ,

P 2 =(3) 2 (7 . 5kW)=68kW .