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It is a classic experiment demonstrating convection. I understand that convection takes place and heats up the air. The heated air rises and pushes the spiral from the bottom. However, how does the rising air cause the spiral to spin, instead of just pushing the whole spiral upwards?

I guess it's related to the shape of the spiral but I am not sure how it works.

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The spiral shape makes it behave like a windmill. The hot air raised and hits the inclined surface of the spiral. There momentum transfer to the spiral can be decomposed in two parts: one along the vertical, which pushes slightly the spiral upwards and one along the horizontal, which creates torque and makes the spiral rotate.

As the spiral is attached to a string, even a tiny torque makes it rotate.

On the other hand, in order to make the spiral move upward, a force greater than the weight of the spiral is needed. It is not realistic to achieve that with a candle, but with, say, a leafblower, you definitely will.

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