028 Can you blow your own sail?


Basically, the air gave the cart a backward force when they are pushed forward by the fan, and a forward force when they collide into the sail.  However, since these two forces need not have the same magnitude (they are not action-reaction pair), it is unclear which direction the net force will be.

The principle of conservation of momentum can make the analysis easier. Just use the fact that the total momentum of the cart plus the air as a system must remain as zero!

With the original sail, the air got pushed out around the sides of the sail. Since the net change in momentum of the air is zero, the cart cannot have any change in momentum.


With the improvised sail, the air got turned around and back. Since the air have acquired a backward momentum, the cart must acquire a forward momentum.


In practice of course it will be silly to blow your own sail forward. It is much easier (and more efficient) to simply blow backward.


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