Question

已知两恒力F₁(3,4)、F₂(6,-5)作用于同一质点,使之由A(20,15)移动到点B(7,0),

试求:(1)F₁、F₂分别对质点所做的功;

(2)F₁、F₂的合力F对质点所做的功.

Answer 1

解析：=(7,0)-(20,15)=(-13,-15),

(1)W₁=F₁·=(3,4)·(-13,-15)=3×(-13)+4×(-15)=-99(焦),

W₂=F₂·=(6,-5)·(-13,-15)=6×(-13)+(-5)×(-15)=-3(焦).

(2)W=f·=(F₁+F₂)·=［(3,4)+(6,-5)］·(-13,-15)=(9,-1)·(-13,-15)=9×(-13)+(-1)×(-15) =-117+15=-102(焦).