88.1 Work done by variable force @the work done by a force depends on the choice of the reference frame. Elevator: W=0, Earth:W≠0 @the work done by a pair of forces is not always zero. M N +形 N 0 W,+W<0 88.1 Work done by variable force 5. The work done by the total force The work done by the total force acting on the system is the algebraic, scalar sum of the work done by the individual forces total ∫m(丙++ F1·d+F2dr+ W,+W,+4 3the work done by a force depends on the choice of the reference frame. Elevator:W=0, Earth: W≠0 y W g r v r + ′ = 0 W N W N N r ′ v c N r v m f c r ′ s s ′ M + ′ < 0 W f W f f r m §8.1 Work done by variable force 4the work done by a pair of forces is not always zero. 5. The work done by the total force The work done by the total force acting on the system is the algebraic, scalar sum of the work done by the individual forces. §8.1 Work done by variable force L L r r r r r L r r r r = + + = ⋅ + ⋅ + = ⋅ = + + ⋅ ∫ ∫ ∫ ∫ 1 2 1 2 total total 1 2 d d d ( ) d W W F r F r W F r F F r f i f i f i