Which of the following statements is incorrect?

Options

A. When a material is under tensile stress, the restoring forces are caused by interatomic attraction while under compressional stress, the restoring force is due to interatomic repulsion.
B. The stretching of a coil is determined by its shear modulus.
C. Rubber is more elastic than steel.
D.  Shearing stress plays an important role in the buckling of shafts.

Explanation

To determine which of the following statements is incorrect, we will analyze each option one by one:

### Step 1: Analyze Option A
**Statement:** When a material is under tensile stress, the restoring forces are caused by interatomic attraction while under compressional stress, the restoring force is due to interatomic repulsion.

**Analysis:**
- Under tensile stress, the material is stretched, and the atoms are pulled apart. The restoring force that tries to bring them back to their original position is due to interatomic attraction.
- Under compressional stress, the material is compressed, and the atoms are pushed closer together. The restoring force in this case is due to interatomic repulsion as the atoms try to return to their equilibrium positions.

**Conclusion:** Option A is **correct**.

### Step 2: Analyze Option B
**Statement:** The stretching of a coil is determined by its shear modulus.

**Analysis:**
- The stretching of a coil (or spring) is primarily determined by its spring constant (k), which is related to Young's modulus rather than shear modulus. Shear modulus pertains to how materials deform under shear stress, not tensile stress.

**Conclusion:** Option B is **incorrect**.

### Step 3: Analyze Option C
**Statement:** Rubber is more elastic than steel.

**Analysis:**
- Elasticity is often measured by Young's modulus. Rubber has a lower Young's modulus compared to steel, meaning for the same amount of stress, rubber will deform more than steel. However, this does not mean rubber is more elastic; rather, it means it is more deformable under stress. Steel is considered more elastic because it can return to its original shape more effectively after the stress is removed.

**Conclusion:** Option C is **incorrect**.

### Step 4: Analyze Option D
**Statement:** A shearing stress plays an important role in the buckling of a shaft.

**Analysis:**
- Buckling of a shaft is primarily influenced by compressive stress rather than shear stress. However, shear stress can contribute to the overall deformation of the shaft under certain conditions, but it is not the primary factor in buckling.

**Conclusion:** Option D is **correct**.

### Final Conclusion
After analyzing all options, we find that:
- **Option B** is the incorrect statement regarding the stretching of a coil being determined by its shear modulus.

### Summary of Incorrect Statement:
- **Incorrect Statement:** Option B: The stretching of a coil is determined by its shear modulus.