A copper ball easily slides through a copper ring when it is cold but after there has been heated it does not fit through the ring anymore.
What happens to the ball during heating..?
During heating, the copper ball expands due to an increase in its temperature. This expansion causes its diameter to increase. Therefore, when it cools down, its diameter reduces to its initial size. However, the copper ring also expands when it is heated but not as much as the ball because of its lesser volume. Therefore, when the ball is attempted to pass through the hot ring, the reduced gap between their diameters makes it difficult for the ball to move through. As a result, the ball does not fit through the ring anymore after heating.
When the copper ball is heated, it undergoes thermal expansion. This means that its particles gain energy and start vibrating more vigorously, causing the ball to expand and increase in size. As a result, the diameter of the ball increases.
Now, let's consider the copper ring. When the ring is cold, its inside diameter is slightly smaller than the diameter of the cold ball. However, during heating, the ring also expands due to thermal expansion but not as much as the ball.
Since the ball expands more than the ring, its increased diameter prevents it from fitting through the ring after heating. The once-easy sliding motion is no longer possible due to the size mismatch caused by the differential thermal expansion of the copper ball and ring.