Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: The work done by static friction on a body can be positive.
Reason R: Static friction on a body always acts opposite to the direction of motion of the body.
In the light of the above statements, choose the correct answer from the options given below:
Answer: (D) A is true but R is false
Concept. Static friction opposes the relative motion that would occur between the surfaces, not the motion of the body itself.
Step 1. Assertion. A box resting on the floor of a truck that speeds up is carried forward by static friction, which acts forward along its displacement. The work done on the box is positive. A is true.
Step 2. Reason. In that same example static friction acts along the box's motion, so it does not always oppose the motion. R is false.
Answer. A is true but R is false (Option D).
Solution by Sreeraj P, M.Sc Physics