explain xkcd - User contributions [en]

3090: Sail Physics

2025-05-16T23:45:12Z

Leo r:

{{comic
| number = 3090
| date = May 16, 2025
| title = Sail Physics
| image = sail_physics_2x.png
| imagesize = 699x263px
| noexpand = true
| titletext = Turning in other directions can be accomplished by using a magnetized centerboard and ocean currents, since a current flowing through a magnetic field induces a Laplace force.
}}

==Explanation==
{{incomplete|This page was created by a wind-blown electron. Don't remove this notice too soon.}}

This comic shows what initially appears to be an explanation of how boats sail upwind. However, it quickly devolves into wrongness, where, instead of even mentioning {{w|tacking (sailing)|tacking}}, it dives into a totally wrong{{citation needed}} explanation involving electrons and magnetism.

This humor works at another level {{citation needed}} - all\* interaction of physical things at macro scale (humans and boat sized objects) are of electromagnetic in nature (this is considering, that at this scale, among the 4 fundamental interactions, only electromagnetic interactions are of reasonable strength {{citation needed}}). So one unaware of sailing mechanics may start to explain the situation with electromagnets, and could come to this line of thinking, but it is wrong. If we are to consider this, {{w|Triboelectric effect|Triboelectric effect}} is the mechanism of acquisition of static charge. Though, it depends on relative frictional forces invloved between two objects, which inturn depends on effective interaction surface area. But the charge acquired would be very small, and not be of comparitive order of magnitudes with gravity. There is more charge generated due to running of water across the boat in a realistic situation {{citation needed}}. Then the comic assumes motion in direction of downstream, due to effective normal forces on the sail. Then the comic considers this motion as motion of charged object in Earth's magnetic field. If we are to consider this magnetic field to be roughly parallel to Earth surface, which would be true for situations close to equator (as Earth's magnetic field is effectively that of dipole bar magnet, which is roughlly aligned with our planetary rotational axis (as of writing of this explaination, as the magnetic field dipole keeps on maoving, and is also expected to do a complete flip in a few 10000 years, so there would be some time where it is in the equitorial plane at some point (assuming a continous motion of Earth's dipole))). In which case, the net lorentz force on the boat would be $ F = q (v \times B) $, and since both velocity of both and magnetic field lines are in thae same plane (as viewed in the above sketches, a top-down manner) then the force would be perpendicular to plane, that is, parallel to radius vector towards Earth's core, so it will not cause any motion in plane due to this force. If we consider magnetic field lines to be perpendicular to Earth's surface (close to poles), even then the magnitude of the force is very small. If we consider a net charge of 1 Coloumb across the boat (which is a extremely high amount of charge, at which point, the charge would try to acumulate on sharp objects, and due to this high charge density, the sharp objects would start to break of from the boat (this is due to high self electrostatic energy, which is roughly proprtional to square of charge density divided by radius of curvature of object), assuming that this charge does not cause the ship to break or the charge to leak out to water body), and also consider field strength of order $ 10^(-4) $ Tesla (close to typical values on surface {{citation needed}}), and take wind speeds of 10000 m/s (also very high {{citation needed}}) then the net force on ship would be *1 N*, which for a boat of mass 10 Kg (less for a typical boat, but is appropriate for a makeshift raft {{citation needed}}) cause a net acceleration of 0.1 m/s^2. Which is not extremely small, but due to friction across the boat, would be cancelled (by 1 or 2 orders of magnitude {{citation needed}}).

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

[Panel 1]

How sailboats use physics to sail upwind:

[A schematic boat with a sail is shown (top-down view). Winds shown with directional arrows (going towards 4:30 on a clock face).]

[Panel 2]

1. Wind passing over the sail strips away electrons via the triboelectric effect.

[Schematic similar to panel 1, but with charged ions shown across both sides of the sail]

[Panel 3]

2. The positively charged boat is blown downwind; its movement in Earth's magnetic field produces a Lorentz force.

[A force vector is shown in direction of wind, and a perpendicular force vector (along 1:30 on watch face) is shown with a dashed arrow.]

[Panel 4]

3. The Lorentz force acts perpendicular to the direction of motion, redirecting the boat upwind.

[A net force vector is shown perpendicular to downstream vector.]

{{comic discussion}}<noinclude>

3090: Sail Physics

2025-05-16T23:44:54Z

Leo r:

{{comic
| number = 3090
| date = May 16, 2025
| title = Sail Physics
| image = sail_physics_2x.png
| imagesize = 699x263px
| noexpand = true
| titletext = Turning in other directions can be accomplished by using a magnetized centerboard and ocean currents, since a current flowing through a magnetic field induces a Laplace force.
}}

==Explanation==
{{incomplete|This page was created by a wind-blown electron. Don't remove this notice too soon.}}

This comic shows what initially appears to be an explanation of how boats sail upwind. However, it quickly devolves into wrongness, where, instead of even mentioning {{w|tacking (sailing)|tacking}}, it dives into a totally wrong{{citation needed}} explanation involving electrons and magnetism.

This humor works at another level{{citation needed}} - all\* interaction of physical things at macro scale (humans and boat sized objects) are of electromagnetic in nature (this is considering, that at this scale, among the 4 fundamental interactions, only electromagnetic interactions are of reasonable strength {{citation needed}}). So one unaware of sailing mechanics may start to explain the situation with electromagnets, and could come to this line of thinking, but it is wrong. If we are to consider this, {{w|Triboelectric effect|Triboelectric effect}} is the mechanism of acquisition of static charge. Though, it depends on relative frictional forces invloved between two objects, which inturn depends on effective interaction surface area. But the charge acquired would be very small, and not be of comparitive order of magnitudes with gravity. There is more charge generated due to running of water across the boat in a realistic situation {{citation needed}}. Then the comic assumes motion in direction of downstream, due to effective normal forces on the sail. Then the comic considers this motion as motion of charged object in Earth's magnetic field. If we are to consider this magnetic field to be roughly parallel to Earth surface, which would be true for situations close to equator (as Earth's magnetic field is effectively that of dipole bar magnet, which is roughlly aligned with our planetary rotational axis (as of writing of this explaination, as the magnetic field dipole keeps on maoving, and is also expected to do a complete flip in a few 10000 years, so there would be some time where it is in the equitorial plane at some point (assuming a continous motion of Earth's dipole))). In which case, the net lorentz force on the boat would be $ F = q (v \times B) $, and since both velocity of both and magnetic field lines are in thae same plane (as viewed in the above sketches, a top-down manner) then the force would be perpendicular to plane, that is, parallel to radius vector towards Earth's core, so it will not cause any motion in plane due to this force. If we consider magnetic field lines to be perpendicular to Earth's surface (close to poles), even then the magnitude of the force is very small. If we consider a net charge of 1 Coloumb across the boat (which is a extremely high amount of charge, at which point, the charge would try to acumulate on sharp objects, and due to this high charge density, the sharp objects would start to break of from the boat (this is due to high self electrostatic energy, which is roughly proprtional to square of charge density divided by radius of curvature of object), assuming that this charge does not cause the ship to break or the charge to leak out to water body), and also consider field strength of order $ 10^(-4) $ Tesla (close to typical values on surface {{citation needed}}), and take wind speeds of 10000 m/s (also very high {{citation needed}}) then the net force on ship would be *1 N*, which for a boat of mass 10 Kg (less for a typical boat, but is appropriate for a makeshift raft {{citation needed}}) cause a net acceleration of 0.1 m/s^2. Which is not extremely small, but due to friction across the boat, would be cancelled (by 1 or 2 orders of magnitude {{citation needed}}).

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

[Panel 1]

How sailboats use physics to sail upwind:

[A schematic boat with a sail is shown (top-down view). Winds shown with directional arrows (going towards 4:30 on a clock face).]

[Panel 2]

1. Wind passing over the sail strips away electrons via the triboelectric effect.

[Schematic similar to panel 1, but with charged ions shown across both sides of the sail]

[Panel 3]

2. The positively charged boat is blown downwind; its movement in Earth's magnetic field produces a Lorentz force.

[A force vector is shown in direction of wind, and a perpendicular force vector (along 1:30 on watch face) is shown with a dashed arrow.]

[Panel 4]

3. The Lorentz force acts perpendicular to the direction of motion, redirecting the boat upwind.

[A net force vector is shown perpendicular to downstream vector.]

{{comic discussion}}<noinclude>