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A fan and a sail are both mounted vertically on a cart resting on a horizontal table, with the fan blowing an air stream toward the right onto the sail. The cart can move with negligible friction. After the fan is switched on, the cart will

1. move to the right and then to the left
2. remain at rest
3. move towards the right
4. move towards the left

Correct answer: move towards the left

Solution

The fan pushes air right and recoils left; if the sail caught and stopped all the air the forces would cancel, but since air rebounds/escapes from the sail, the net reaction leaves the cart moving toward the left (toward the fan side).

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