You have a bag of cotton and an iron bar, indicating mass of `100` kg, when measured on a weighing machine . In reality

Options

A. cotton is heavier than iron
B. cotton and iron are equally heavy
C. iron is heavier than cotton
D. none of the above

Explanation

To solve the question regarding the comparison of the weight of a bag of cotton and an iron bar, we will analyze the concepts of buoyancy and weight measurement.

### Step-by-Step Solution:

1. **Understanding the Weighing Machine Reading**:
- The weighing machine shows a mass of `100 kg` for both the cotton and the iron bar. This means that when measured, both objects exert the same force on the scale.

2. **Concept of Buoyancy**:
- Buoyancy is the upward force exerted by a fluid (in this case, air) on an object immersed in it. The buoyant force depends on the volume of the object and the density of the fluid.
- The formula for buoyant force (Fb) is:
\[ Fb = \text{Density of fluid} \times \text{Volume of object immersed} \]

3. **Comparing the Volume of Cotton and Iron**:
- Cotton is less dense than iron, which means for the same mass, cotton occupies a larger volume than iron.
- Since the volume of cotton is greater, the buoyant force acting on the cotton will also be greater than that acting on the iron.

4. **Effect of Buoyant Force on Weighing Machine Reading**:
- When the cotton is weighed, the buoyant force reduces the effective weight that the scale measures. Therefore, to balance the greater buoyant force acting on the cotton, we need to have a greater actual mass of cotton.
- If both are indicated as `100 kg` on the weighing machine, the actual mass of the cotton must be greater than `100 kg` to compensate for the buoyant force.

5. **Conclusion**:
- In reality, the cotton is heavier than the iron bar. Therefore, the correct answer to the question is that "cotton is heavier than iron."

### Final Answer:
Cotton is heavier than iron.

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