两个力F1=(1,1) F2=(4,-5)作用于同一个质点 使该质点从A(20,15)移到点B(7,0)
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B - A =(7,0)-(20,15) = (-13,-15)

1).F1,F2分别对质点所做的功

A1 = (1,1)· (-13,-15) = -13 - 15 = - 28

A1 = (4,-5)· (-13,-15) = -52 + 75 = 23

2).F1,F2的合力F对质点所做的功

F1 + F2 = (1,1) +(4,-5)= (5,-4)

A = (5,-4)· (-13,-15) = -65 + 60 = -5

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