Question

Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : When a rod lying freely is heated, no thermal stress is developed in it.
Reason R : On heating, the length of the rod increases. In the light of the above statements, choose the correct answer :

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Stress requires a restoring force. If a rod expands freely, there is no force pushing back, hence no stress.

Answer 1

The Correct Option is B

Step 1: Thermal stress is only produced when the thermal expansion or contraction of a body is prevented by external constraints.
Step 2: A rod lying freely can expand without any resistance, so stress is zero. Assertion A is true.
Step 3: Heating leads to expansion ($\Delta L = L \alpha \Delta T$). Reason R is true.
Step 4: However, the expansion itself isn't the reason for zero stress; the lack of constraint is.