# Action-reaction of a sailing boat and wind

• B
• kirito
kirito
compare 2 situations When a person stands inside a boat and blows air on the sail will the boat move or not and second case human blowing but outside of the boat ,

thought of something along the lines of
the human exerts force on air , air applies equal but opposite force on the human , the human is on the boat so it applies a force on the boat ground forward , the air applies force on the sail but the sail is attached to the boat so it applies a force backwards so the boat does not move ,

it is easy to see that something is wrong my whole analogy since if no one was in the boat and the air pushed the boat forwards it will moves in that direction and if it will apply a force it will be on the water backwards ,

I tried of thinking of human and boat as a single quantity and boat and sail as one so human and boat should move backwards because of air but boat and sail forward so the boat is not moving

If a clear identification of how to use newtons third law and analogy on what I did wrong can be provided , pardon and thanks

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kirito said:
thought of something along the lines of
the human exerts force on air , air applies equal but opposite force on the human , the human is on the boat so it applies a force on the boat ground forward , the air applies force on the sail but the sail is attached to the boat so it applies a force backwards so the boat does not move ,
Your reasoning is mostly correct, but note that the boat doesn't move if the two forces in opposite directions are equal in magnitude. They are not equal, so the boat can move. The force you exert on the air is equal to the force the air exerts on you by Newton's third law. The force the air exerts on the sail and the force the sail exerts on the air are equal by Newton's third law. But the first pair are not necessarily equal to the second pair.

What happens is that your stream of breath hits the sail and reverses, and flows back past you. Ideally, the effect is just the same as standing on the boat and blowing in the opposite direction. In practice the two streams of air moving in opposite directions will disrupt each other and you lose efficiency by using a sail.

kirito
Ibix said:
Your reasoning is mostly correct, but note that the boat doesn't move if the two forces in opposite directions are equal in magnitude. They are not equal, so the boat can move. The force you exert on the air is equal to the force the air exerts on you by Newton's third law. The force the air exerts on the sail and the force the sail exerts on the air are equal by Newton's third law. But the first pair are not necessarily equal to the second pair.

What happens is that your stream of breath hits the sail and reverses, and flows back past you. Ideally, the effect is just the same as standing on the boat and blowing in the opposite direction. In practice the two streams of air moving in opposite directions will disrupt each other and you lose efficiency by using a sail.
and regarding the the sail and the boat as a whole should I think of the force Applied on the sail as applied to the boat itself since they are attached and so that when the sail is pushed forward so is the boat and it pushes water pushes the water backwards the water causes it to move forward , since when I think about either sail and boat or human on boat I get confused

for example I start thinking is it the opposite the sail pushed forward it pushes boat backwards which pushes the water forward and water pushes back and seems faulty intuitively or the human being pushed back so he is pushing the boat forward and boat pushes water backwards and water pushes boat forward...

or is the human being pushed back with the boat pushing the water forward and all being pushed forward by the water backwards which feels to make much more since , is this depending on if there is a static friction holding them together or not and sorry for the long comment

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Ibix said:
Your reasoning is mostly correct, but note that the boat doesn't move if the two forces in opposite directions are equal in magnitude. They are not equal, so the boat can move. The force you exert on the air is equal to the force the air exerts on you by Newton's third law. The force the air exerts on the sail and the force the sail exerts on the air are equal by Newton's third law. But the first pair are not necessarily equal to the second pair.

What happens is that your stream of breath hits the sail and reverses, and flows back past you. Ideally, the effect is just the same as standing on the boat and blowing in the opposite direction. In practice the two streams of air moving in opposite directions will disrupt each other and you lose efficiency by using a sail.
like when I think of a block of mass a and a block of mass b on the ground on top of each other I know that when I apply a force on block a block a applies same force on me but that does not mean it will apply it on b or the ground what will be applied on the ground is the force from both of them touching and same for and b , also if there is friction between them friction on each other but the force I applied is not being applied on that upper block , yet here I seem to be going on a circle between analysing how many newton pairs are or if I am adding pairs that finished effecting the system earlier

kirito
kirito said:
thanks helped in visualising the event in a clearer manner
PeroK said:

kirito said:
it is easy to see that something is wrong my whole analogy since if no one was in the boat and the air pushed the boat forwards it will moves in that direction
But the source of the wind is not in the boat, so that analogy fails.

For the case of the source being aboard the boat, it all depends on the shape of the sail. If the sail can reverse the direction of the air flow you end up with air being propelled towards the stern.

Engineers employ this strategy to slow down a jetliner landing on a runway. Jets that propell air towards the rear of plane have cowls inserted in the airstream that direct the airstream towards the front of the plane, helping to slow the plane down.

kirito
Mister T said:
But the source of the wind is not in the boat, so that analogy fails.

For the case of the source being aboard the boat, it all depends on the shape of the sail. If the sail can reverse the direction of the air flow you end up with air being propelled towards the stern.

Engineers employ this strategy to slow down a jetliner landing on a runway. Jets that propell air towards the rear of plane have cowls inserted in the airstream that direct the airstream towards the front of the plane, helping to slow the plane down.

kirito said:
It's not applied in practice for sail boats, because having a fan on a sailboat would defeat the point of the sail, as the fan could the propel the boat more efficiently by itself. But it can work just like thrust reversal on airplanes, as the video shows.

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kirito

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