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We're a bunch of friends building an RC boat, this may sound stupid but I had an argument with a friend where he insisted that his design will work.

Basically, he's saying that by putting a powerfull fan and a piece of tissue as a sail, when the fan will blow air into it, the boat will move.

What do you think?

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    $\begingroup$ That if the fan is on the boat, it will not work. Unfortunately, pushing the dashboard in your car does not make you car move faster, neither. $\endgroup$
    – anderstood
    Commented Apr 27, 2015 at 18:05
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    $\begingroup$ This most certainly will not work the way your friend thinks. Skip the tissue and direct the airflow in the direction opposite to your preferred travel direction and you might be able to accomplish a descent airboat. $\endgroup$
    – jkej
    Commented Apr 27, 2015 at 18:14
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    $\begingroup$ Yes it will work, but (a) not very efficiently and (b) someone else got there first! $\endgroup$
    – John Rennie
    Commented Apr 27, 2015 at 18:15

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Newton can answer this one - equal and opposite forces.

The air hitting the tissue will exert force pushing the boat forwards, but at the same time the fan is exerting a roughly equal force pushing the boat backwards (actually a little greater, since not all of the air hits the tissue).

Think of the fan like how a propeller pushes an airplane.

Like @jkej mentioned in their comment, forget the sail and make an RC airboat instead. :)

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Theoretically this will work. But to understand why this will work, you need to see what won't work. Here is a diagram which won't work: enter image description here This is a closed system and assuming that air particles are not accelerating, there won't be any external force in horizontal direction and thus boat can't move.

Now take a look at your diagram. Obviously air particles are accelerating and wind is rushing towards the boat. Once the wind hits the tissue, it will get scattered uniformly in all directions thereby causing net horizontal force zero. Refer the figure for a proper understanding:

enter image description here

Your model looks somewhat like this. Now F1 and F2 might happen to cancel each other but F3 will remain there as an external force.

That's why the boat will move.

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