Question

Assertion :All surfaces shown in figure are smooth. Block A comes down along the wedge B. Work done by normal reaction (between A and B) on B is positive while on A it is negative. Reason: Angle between normal reaction and net displacement of A is greater than ${90}^o$ while between normal reaction and net displacement of B is less than ${90}^o$.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

Answer: (A)

The normal reaction acting on A is perpendicular to its surface and into the wedge, and the displacement of A is along the horizontal towards the left. Thus the angle between normal reaction and displacement is less than ${90}^{\circ }$. Hence the work done is positive.

The normal reaction acting on B is perpendicular the wedge and towards the block B. The displacement of block B has two components, one along the wedge downwards with respect to the wedge, and other horizontally towards left, the displacement of the wedge. The vector addition of these two displacements give the net displacement. Thus the angle between the displacement and normal reaction is more than ${90}^{\circ }$ and hence work done is negative.