Iron rims are heated red hot before planting on car wheels. Why?

Answer

**Step-by-Step Solution:**

1. **Understanding the Initial Condition**: The iron rims have a radius that is slightly smaller than the radius of the car wheels. This means that the rims cannot fit onto the wheels in their initial state.

2. **Heating the Rims**: To make the rims fit onto the wheels, they are heated until they are red hot. Heating the rims increases their temperature.

3. **Thermal Expansion**: As the temperature of the iron rims increases, they undergo thermal expansion. This means that the material of the rims expands, causing their radius to increase.

4. **Fitting the Rims**: Once the radius of the heated rims expands sufficiently, they can now fit onto the wheels easily. The expansion allows the rims to slide over the wheels without any obstruction.

5. **Cooling the Rims**: After the rims are placed onto the wheels, water is poured over them. This action cools the rims down, decreasing their temperature.

6. **Contraction**: As the temperature of the rims decreases, they undergo thermal contraction. The radius of the rims decreases back to a size that fits snugly onto the wheels.

7. **Final Fit**: The contraction ensures that the rims are firmly fixed onto the wheels, providing a secure fit.

**Conclusion**: Iron rims are heated red hot before planting on car wheels to allow for thermal expansion, making it possible for the rims to fit onto the wheels. After fitting, cooling causes the rims to contract and secure their position.

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