0
$\begingroup$

So we've got the classic physics problem with the fan powered sail. If we have a boat with a sail, and we attach a fan onto it, facing the sail, the boat will not move due to the balance of forces:

The fan pushes air forward and the air pushes the fan backward, thus pushing the boat backward. But air also pushes forward on the sail as the sail pushes backward on the air, so since the sail is ALSO attached to the boat, the boat is being pushed both forward and backward by air. Thus, no motion. (does this explanation seems accurate?)

But I was wondering. What happens if you add a second fan? Does that mean you have a greater force pushing backward on the boat compared to the force pushing forward on the boat? So adding a second fan causes the boat to move backward? Or does it not matter what strength the fan is? I think I'm getting confused because something about my above explanation is off, and I'm comparing the wrong forces.

$\endgroup$
2
  • $\begingroup$ However, a curved sail changes it all. $\endgroup$
    – FGSUZ
    Jul 25 '18 at 21:14
  • $\begingroup$ Why do you think adding a fan would change the situation? $\endgroup$ Jul 27 '18 at 20:42
2
$\begingroup$

If you add a second fan, the boat still won't move (using this model). You will have twice the air pressure force acting on the sail, but also two fans that are pushing backward on the boat, so the forces will still balance.

$\endgroup$
2
  • $\begingroup$ Okay then that's in line with my original intuition. Thanks so much! $\endgroup$
    – wildlemon
    Jul 25 '18 at 22:15
  • 1
    $\begingroup$ Take the sail down and point both fans in the other direction! $\endgroup$ Jul 26 '18 at 0:23
1
$\begingroup$

If you point the second fan at the sail as well, it would contribute no net force, the same way the first fan does.

If you point the fan somewhere else, you have an airboat; but you could do that by just getting rid of the sail in the first place.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.