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问题补充：

答案：

(a) Assuming that each stroke is a perfect half circle,the distance of each stroke is L=½πD=0.5x3.14x1.8=2.83 m.

Therefore,the work done in each stroke is F x L=260N x 2.83m=734.8 J.

(b) In each stroke,the upward and downward force is not useful in pulling him along.

The useful work done in each stroke is F x D=260N x 1.8m=468 J.

Time of each stroke 60/110=0.545 s.

Therefore,the useful power is 468 / 0.545 = 858.0 W.

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