[gem (OP)]

Hi all,

So Colin has been sailing recently, So not only is he good at physics he will also have been experiencing natures laws and forces first hand.

Would anyone like to venture an answer.
 

[Origin]

Wind.

[evan_au]

Entropy.

A mass of air moving at one velocity, in close proximity to a mass of water moving at a different velocity.

Colin's sailing boat sits at the interface of these two masses - after the boat passes, the air is moving a bit slower, and the water is moving a bit faster. The difference in velocity between these two masses is less.

Colin's enjoyment of the world has increased, as has the entropy of the world.

[Bored chemist]

I read of some rather funny claim that it was due to the spin of the Earth, but it's generally going to be solar powered.

[alancalverd]

I do hope he didn't make the mistake of using a flat sail when travelling crosswind.

[gem (OP)]

Hi all,

So Evan_au

A mass of air moving at one velocity, in close proximity to a mass of water moving at a different velocity.

Colin's sailing boat sits at the interface of these two masses - after the boat passes, the air is moving a bit slower, and the water is moving a bit faster. The difference in velocity between these two masses is less.

Colin's enjoyment of the world has increased, as has the entropy of the world.

Now lets not get to nautical, but i'm sure Colin may correct you on sailing and the importance of tide tables, especially if he's been sailing in
"The Solent"  and going with the tidal current.

However if Origin and BC are correct wouldn't the entropy/transformation. be reversed the next day ?

Origin

Wind.

BC

but it's generally going to be solar powered.

[alancalverd]

Wind and tide may change but entropy doesn't generally get reversed. In fact the summary of life, the universe, and everything, is ΔS > 0. 

[gem (OP)]

Hi all
yes thanks Alan, a poor use of word on my part, I probably should have said replenished.

So doesn't the continued increase of entropy of the Earth system due to Colin's sailing activities continually decrease also due to the solar input ?   

[alancalverd]

No.

[Bored chemist]

However if Origin and BC are correct wouldn't the entropy/transformation. be reversed the next day ?

The entropy change might be happening in the Sun...

[gem (OP)]

Hi all

Alan

No

Perfect, 

So we are agreed Colin's sailing boat actual physical processes that gives him his "speed over ground" are not symmetric in time ?
Such as the heeling over that occurs, when the wind billows his sails full and the tell tails go horizontal and the ropes groan under tension, similar to the flapping and rustling of foliage on dry land.     

[alancalverd]

You could look at a simpler model. The motion of a boat through moving water is depressingly complicated, but if we just look at the sail as an aerofoil you can see that wind can produce a force on the foil, and waving a sail can produce wind. Flapping sails aren't a lot of use, but propellors and windmills are simply time-reversals of the same Bernouilli effect.

Incidentally if BC is awake (does he ever sleep?) he might ponder why a boat can sail faster across the wind than it can downwind, if the underside of an aerofoil is the only surface that contributes lift.

[gem (OP)]

Hi all,
Alan in regards to the faster than the  wind I believe there is a post already live on that issue, here on tns.(something about a bet was in the media I believe)

On the other point of reversibility and time symmetry I don’t want to get to in depth if possible but a general agreement as to the fact of heat input and friction are involved should do away with Bernoulli effect as a counter argument, as to what’s actually occurring in reality.
😊

[gem (OP)]

Hi all,

So lets assume as suggested the source of the kinetic energy is the Suns electromagnetic radiation and lets also assume a speed over ground of 4 m/s therefore for every kilogram mass of the boat we have
kinetic energy of 8.0 Joules and corresponding momentum of 4.0 Kg m/s

Now if we consider the energy and momentum force of the electromagnetic radiation received at the earths surface,
so for ease lets approximate the intensity at       1KW /m^2  that is 1000 J/s
 
and this gives a momentum pressure of between 3.33 x 10^-6 N/m^2  and 6.67 x 10^-6 N/m^2
depending on full absorption to perfect reflection, of the light energy received.

So why is there such disparity between the two sets of figures for the value of momentum ? 
where did the mass of the boat get this momentum from.

[alancalverd]

The earth has an awful lot of square meters compared with a boat.

[evan_au]

lets assume as suggested the source of the kinetic energy is the Suns electromagnetic radiation

I think that is a fair assumption.

Light delivers a lot of electromagnetic energy: much of which gets turned into heat, some of which appears as the kinetic energy of circulating air currents and ocean currents.


But then you jump to this:

this (momentum of light) gives a momentum pressure of between 3.33 x 10^-6 N/m^2  and 6.67 x 10^-6 N/m^2

As you observe, the pressure that photons deliver directly to the sail is small.
- That is why solar sails are only useful in space, where there is no friction, and no air currents.

The force of air pushing on the sails is significant, and the force of water pushing on the keel is significant
- But the force of photons pushing on the sails is insignificant

Colin's sailboat doesn't go anywhere when it is becalmed (no air currents), even when the Sun is still shining brightly.

[Bored chemist]

ncidentally if BC is awake (does he ever sleep?) he might ponder why a boat can sail faster across the wind than it can downwind, if the underside of an aerofoil is the only surface that contributes lift.

I did

https://www.thenakedscientists.com/forum/index.php?topic=82462.msg643539;topicseen#msg643539

You can make the idea work with no classic  "aerofoils" at all- you can use flat planks to build the propeller. It won't be efficient, but it doesn't need to be

[alancalverd]

On my planet, a plank still has two sides.

[Eternal Student]

Hi.

and this gives a momentum pressure of between 3.33 x 10^-6 N/m^2  and 6.67 x 10^-6 N/m^2
depending on full absorption to perfect reflection, of the light energy received.

So why is there such disparity between the two sets of figures for the value of momentum ?
where did the mass of the boat get this momentum from.

   Two things seem to be relevant:

1.  Momentum is a vector quantity not a scalar.   Conservation of momentum doesn't state that the total sum of the absolute value of momentum never increases,  only that the vector sum of the momentum remains constant.   This is a lot of words so I'll re-phrase it another way:     
    A big blob of matter might be at rest    @@     but it can suddenly  split into two smaller blobs of matter 
   ←@      @→      where each individual little blob moves away from the other.     
This is perfectly fine provided the  sum of the momentum to the right →  minus the momentum to the left  ←   still  equals 0.
    The absolute momentum has increased BUT the vector sum of momentum has not changed.

2.  The atmosphere can undergo processes where momentum is created,  provided the vector sum of momentum remains constant.   As a consequence the absorption of energy from the radiation of the sun becomes as important as the absorption of the momentum from the radiation.    Energy can be used to apply a force to some part of the atmosphere and keep that force applied over a certain distance.

Sailing is fine pursuit.  I wish you fair winds and calm seas.  Bye for now.

[alancalverd]

Momentum is a vector quantity not a scalar.......the  sum of the momentum to the right →  minus the momentum to the left  ←   still  equals 0.......The absolute momentum has increased

Something of an oxymoron. The sum of the arguments has increased, but as momentum is a vector, that has nothing to do with momentum! You have increased the kinetic energy (a scalar) of the system.

Beware - you could incur the wrath of a bored chemist as well as the raised eyebrow of a rather busy physicist.