# Physics

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Rotating cylinders are not just physical abstractions: for example, they can turn a flow of the air, thus creating lift as the airplane wings. Historically people have even used rotating cylinders to power ships (although with little efficiency). NASA wrote more about this here.

We will now examine how much time we need to stop a ship-engine-like cylinder. Suppose you take a rotating cylinder and let it roll to the right until it reaches a vertical wall. It then spins against the two walls as shown in the picture, slowly slowing down. How much time will it take in s for the cylinder to stop?

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