Question

In order to fit a metal rim to the wooden wheel of a bullock cart, the metal rim is ______________________________________ and placed on the wooden wheel and ______________________________________. This process works because ______________________________________.

• A. Heated, cooled, the metal expands as it cools
• B. Cooled, heated, the metal shrinks as it cools
• C. Heated, cooled, the metal shrinks as it cools
• D. Cooled, heated, the metal expands as it cools

Hint:

Remember the basic principle of thermal physics: Heat causes expansion, and cooling causes contraction. This is a fundamental concept often applied in engineering and everyday phenomena.

Answer 1

The Correct Option is C

This trick is the same one used for tightening a loose metal lid on a glass jar or fitting a metal ring onto an axle: heat the metal part so it grows slightly bigger, slide it into place while it is still hot, then let it cool.

As the rim cools back down to room temperature, it contracts, and since the wooden wheel underneath does not shrink along with it, the rim ends up gripping the wood far more tightly than if it had simply been hammered on cold.

This whole process depends on one simple fact, metals expand when heated and shrink when they cool back down, which is why the rim has to be heated first and cooled only after it is in position.

That sequence, heat then cool, with shrinking on cooling as the reason it grips, is exactly what makes the fit work.

Answer 2

Another way to reach the same answer is to reason about what would go wrong if the process were done in the wrong order, rather than starting from the correct sequence directly.

1. Heated, cooled, the metal expands as it cools: if the metal really expanded on cooling, the rim would keep growing after fitting and eventually crack the wooden wheel, which is not what happens in practice, so this reasoning fails.
2. Cooled, heated, the metal shrinks as it cools: cooling the rim before fitting would make it smaller and even harder to slip over the wheel, and there would be no reason to heat it afterward once it is already in place, so this order does not serve any purpose.
3. Heated, cooled, the metal shrinks as it cools: heating first lets the rim slide easily over the wheel, and shrinking on cooling afterward is exactly the mechanism that pulls the rim in tight around the wood, which is the actual purpose of the whole process.
4. Cooled, heated, the metal expands as it cools: heating the rim only after it is already fitted would make it expand and loosen its grip rather than tighten it, defeating the purpose of the technique.

Only heating first, cooling after, with shrinkage on cooling as the working principle, actually achieves a tight fit, which is why that option is correct.

Therefore, the correct answer is Heated, cooled, the metal shrinks as it cools.