The Total solution for NCERT class 6-12
The sign of work done by a force on a body isimportant to understand. State carefully if the following quantities arepositive or negative:(a) Work done by a man in liftinga bucket out of a well by means of a rope tied to the bucket,(b) Work done by gravitationalforce in the above case,(c) Work done by friction on abody sliding down an inclined plane,(d) Work done by an applied forceon a body moving on a rough horizontal plane with uniform velocity,(e) Work done by the resistiveforce of air on a vibrating pendulum in bringing it to rest.
Work done, W = T.S = Fs cos θ(a) Work done ‘positive’, because force is acting in the direction ofdisplacement i.e., θ = 0°.(b) Work done is negative, because force is acting against the displacementi.e., θ = 180°.(c) Work done is negative, because force of friction is acting against thedisplacement i.e., θ= 180°.(d) Work done is positive, because body moves in the direction of applied forcei.e., θ= 0°.(e) Work done is negative, because the resistive force of air opposes themotion i.e., θ = 180°.