# Mythbusters: Blow your own sail review

#### JDługosz

The result was surprising. Their "explanation" was nonsense. What is the reason that the boat is able to move forward?

To those who haven't seen it, they put a fan and a sail on a model cart or boat, and as expected, it didn't move. They made the sail very small and it moved backwards: makes sense as some air missed the sail. Having no sail at all is simply the limiting case.

So they made the sail much larger, and it moved forward! His thought is that the sail could not absorb all the force so some of it reflected. What does that even mean?

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#### Pengwuino

Gold Member

Nevermind:

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Isn't this simply explained in terms of momentum.The momentum of the moving air in one direction is equal and opposite to that of the boat.Using a large sail reverses most of the air flow.

#### pyrotix

But the air flow was generated on the boat.

The fan blows the air forwards. This accelerates the boat backwards.

The sail catches the air and captures its momentum. This force is equal and opposite to the force of the fan.

I don't see how this is possible.

#### rcgldr

Homework Helper
The propeller accelerates the air from zero speed to some positive speed. The large sail accelerates the air from that positive speed to some amount of negative speed, so there's more momentum change at the sail.

It would be eaiser to visualize this if the propeller output went into a u shaped tube to redirect the propeller output backwards.

#### pyrotix

Thanks for the clarification. That makes sense. It's a question of how elastic the collision with the sail is.

#### rcgldr

Homework Helper
It's a question of how elastic the collision with the sail is.
The other requirement is that the flow reversed by the sail is far enough away from the propeller's input flow that the sail reversed flow doesn't interefere with the propellers input flow. This is part of the reason that the large sail is needed for this to work.

#### olivermsun

Anyway, if it works, it would work better if you just turned your fan the other way (so that it acts as a propellor).

#### sophiecentaur

Gold Member
The video seems to miss the point and is misleading because what counts is not 'forces and forces cancelling out'. What counts is the direction in which most of the air flows when it's finished interacting with the fan and the sail. It's just Newton III at work, as usual; reaction and action etc..

#### rcgldr

Homework Helper
Note that thrust reversers on jet engines operate on a similar principle. The thrust is reflected forward by the reversers to produce a braking effect on an aircraft.

#### olivermsun

Note that thrust reversers on jet engines operate on a similar principle. The thrust is reflected forward by the reversers to produce a braking effect on an aircraft.
I demand to see this tested on MythBusters! :D

#### Vespa71

Force vectors:
The fan gives force backwards to the boat, by suction and blowing. The sail cancels blowing force by stopping the airflow. It then diverts equal amounts of that air sideways (=0), and some air blows backwards.

Net: Backwards blow force from sail battles inlet suction force, but suction force is smaller, as air is pulled from all directions. So the MB explaination "the sail could not absorb all the force so some of it reflected" holds, kind of, regarding force vectors.

#### sophiecentaur

Gold Member
Since when has "suck" been a force? Pressure is the only force that a gas can apply to an object. It's pressure difference that counts.

#### olivermsun

No, he said it "gives force...by suction and blowing." The suction, referring to a relative low pressure, and blowing, causing a relative high pressure, give a pressure difference and hence the force.

#### Vespa71

Since when has "suck" been a force? Pressure is the only force that a gas can apply to an object. It's pressure difference that counts.
Kinetic energy of air will explain more of the forces here than pressure-difference. (Jet engine?) Then suction is a force. The fan pulls on stationary mass of air.

#### Vespa71

I'm not sure if the fan gives a pressure-difference or simply accelerates air, or both. A pressure difference between the fan and incoming air, and a pressure difference between fan and blowed air, will provide the vectors described. It does not matter how the fan works. The jet-engine example by "rcgldr" also has the same aspect. Suction will accelerate the aircraft, but so much less then the reverse thrust brakes.

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#### sophiecentaur

Gold Member
No, he said it "gives force...by suction and blowing." The suction, referring to a relative low pressure, and blowing, causing a relative high pressure, give a pressure difference and hence the force.
I'm just being precise in a situation where I should have thought precision is important. The word Suck can be very misleading as it implies the existence of a foce that doesn't exist. There is always a way of describing things like this which uses the correct terms and ideas. Better, surely, when someone needs a coherent explanation of a confusing phenomenon.

#### mstacho

So just out of curiosity, how would this result have changed if the boat/sail combination were enclosed and there was zero gravity?

For example: Put the fan and the sail in the middle of the space shuttle or something. Let's assume that the thrusters are cancelling out the rotation induced by the fan, and that the fan and sail are "small" compared to the size of the shuttle's interior, if that's important.

Would the shuttle still move, or would the air eventually hit the walls or something and cancel all the momentum?

Maybe if it works we'll have a new form of space engine

#### pallidin

I don't see how this is possible.
In a closed system there is no way this is possible.
However, the experiment is actually an open system.

#### olivermsun

I'm just being precise in a situation where I should have thought precision is important. The word Suck can be very misleading as it implies the existence of a foce that doesn't exist. There is always a way of describing things like this which uses the correct terms and ideas. Better, surely, when someone needs a coherent explanation of a confusing phenomenon.
If a fan doesn't suck air in one end and blow it out the other, then what does it do in the "correct" terms?

#### rcgldr

Homework Helper
I'm not sure if the fan gives a pressure-difference or simply accelerates air, or both.
At the macroscopic level both. In the immediate vicinity of the propeller, there's almost no change in speed, just an increase in pressure, called the pressure jump. The reduced pressure before the propeller accelerates air towards the propeller, and the increased pressure after the propeller accelerates air away from the propeller.

In this article, the air flow speed across the 'propeller disk' is Vp:

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

So just out of curiosity, how would this result have changed if the boat/sail combination were enclosed and there was zero gravity?
The boat would travel across the inside of the closed system until it hit a wall. There would be circulation of air within the closed system. The center of mass of the closed system would not be affected by any of the internal motions and/or internal accelerations.

#### Fewmet

No, he said it "gives force...by suction and blowing." The suction, referring to a relative low pressure, and blowing, causing a relative high pressure, give a pressure difference and hence the force.
Suppose I pump all the air out of the room on the other side of the wall I am sitting next to. I drill a hole in the wall ad then cover it with my finger. Does the near-vacuum pull the pad of my finger toward the other room, or does air in the room I am in push it that way?

The former seems like describing the change in a room when I turn out the lamp as the darkness moving into the room rather than as the light leaving it.

#### pallidin

Uh... what??

Of course your finger tends to get sucked into that vacuum.
Nothing special here.

#### Fewmet

Uh... what??

Of course your finger tends to get sucked into that vacuum.
Nothing special here.
Really? I'd see it as the air in my room pushing it.

Maybe it is just a quirk of my training, but I always look for a force wherever two bodies are in contact. That leads me to think air molecules push on my finger, not that the absence of a body contacting my finger is pulling it.

If I were floating in a vacuum, would there be no net force on me because the vacuum pulls me equally in all directions? Or is there no force on me because the vacuum applies no force.

#### Vespa71

I'm just being precise in a situation where I should have thought precision is important. The word Suck can be very misleading as it implies the existence of a foce that doesn't exist. There is always a way of describing things like this which uses the correct terms and ideas. Better, surely, when someone needs a coherent explanation of a confusing phenomenon.
You're right.

Ehem.. Force vectors, 2nd take

Suction force can not exist. We'll name it "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller"
The fan gives force backwards to the boat, by "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller" and blowing. The sail cancels blowing force by stopping the airflow. It then diverts equal amounts of that air sideways (=0), some upwards (doesn't matter) and some air blows backwards.

Net: Backwards blow gives force forward to the boat. This battles "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller", but "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller" is smaller, as air is pulled from all directions. So the MB explaination "the sail could not absorb all the force so some of it reflected" holds, kind of, regarding force vectors. They did not describe "The force vector sum of the forces that push on the boat, as a result of a low pressure area behind the boat created by the propeller", presumably to avoid conflicts with scientists when using popular terms to describe the phenomenon.

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