Identify the correct statements from the following :
A. Work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket is negative.
B. Work done by gravitational force in lifting a bucket out of a well by a rope tied to the bucket is negative.
C. Work done by friction on a body sliding down an inclined plane is positive.
D. Work done by an applied force on a body moving on a rough horizontal plane with uniform velocity is zero.
E. Work done by the air resistance on an oscillating pendulum is negative.
Choose the correct answer from the options given below :

Solution

Physics sir · 3 steps

For every statement, ask one thing: does this force point along the motion or against it?

1. Sign of work. For a constant force, W = Fs cos θ. If the force has a part along the displacement, the work is positive. If it points against the displacement, the work is negative. Only a force perpendicular to the displacement, or no displacement at all, gives zero.

   Statement	Force	Displacement	Work
   A	Man's pull, upward	Bucket moves up	Positive, so A is false
   B	Gravity, downward	Bucket moves up	Negative, so B is true
   C	Friction, up the incline	Body slides down	Negative, so C is false
   D	Applied force, along the motion	Body moves forward	Positive, so D is false
   E	Air resistance, against the velocity	Bob moves along its velocity	Negative, so E is true

2. Statement D. Uniform velocity means the net work is zero. The applied force does positive work, and friction does an equal amount of negative work. The work done by the applied force on its own is not zero.
3. So. Only B and E are correct, which is option (d).

The idea to keep. The sign of work comes from the angle between the force and the displacement. Friction and air resistance always oppose motion, so they always do negative work. Zero net work does not mean each force does zero work.

Answer from the NTA final answer key. Solution by PrepWiser.