attraction - T.G.Webb
The other day I was driving into Belfast on a quiet country road.
I am #3 in a line. #1 is travelling at maybe 45 mph, #2 decides to overtake - fair enough. the road is straight and empty ahead. When #2 is right alongside #1, the latter inexplicably drifts to the right to the point where her (I think) wheel is about a foot across the dividing line.
No particular difficulty, the overtaking was completed without incident.
But why the impulse to move over when being overtaken?
I fancy I've encountered this myself when overtaking but here I had a good view of this example, it would have been a good one for a traffic video.
At least #1 didn't speed up, that's very common.
Re: attraction - Alwyn
I forget the scientific name for this, but I know that when ships replenish at sea, (RAS) they have to be careful not to get too close because there is a force or interaction which draws them together.

Come on you scientists, help me here.

Probably lack of concentration on the other drivers part really!
Re: attraction - Piers
It's the same force that gives us Gravity. It's in relation to the mass of the object(s) and the distance from the object.

It's very, very, very small for a car though so I really can't see it being responsible for dragging a car from it's path. Think of the havoc in car parks it could cause....

I'd be more inclined to accept he idea that you steer towards what you are looking at.

Re: attraction - Alwyn

Do you know name of the force. There was a TV programme about the Navy doing a RAS and they mentioned this.. Interaction of....Something........ attraction...... DOH!
Re: attraction - John S

See my post below!

Re: attraction - Piers
I forgot we were talking about moving stuff.... DOH!

As another simple 'amaze someone with science' make a very thick mixture of cornflour and water. Try stiring it quickly. Then get a big pot of the stuff and showing someone how runny it is hold the pot inverted over their head....

Look on the net for the explanations as I can't a: remember b: justify wasting time re-writing what is better written elsewhere.

Re: attraction - JR
As air is forced between the two vehicles it is forced to move faster and causes a reduction in the air pressure. This has same affect as a wing (an aircraft is literally sucked into the air), both cars experience a reduction of air pressure inbetween them and consiquentially there is a tendancy to move together.

Well, thats my understanding of it...

("O" level Physics - 1972)

John R
Re: attraction - Alwyn

The Bernoulli Effect?
Re: attraction - Brian
Partly due to the fact that you tend to focus your attention on the nearer object and subconciously move towards it.
This explains why broken down vehcles on hard shoulders are often hit.
If drivers concentrate on the middle distance the effect is reduced.
Re: attraction - Chad R
It doesn't have to be "between 2 vehicles" any relatively fast moving object will cause a this effect - i.e. why people standing on the hard shoulder get "sucked" into the slow lane when a vehicle passes them at speed. This is accentuated when the moving object is large/heavy and not very aerodynamic i.e. 18 wheeler, and the other object small/light i.e human.

However I'm not sure whether 2 cars (similar mass) only travelling at a differential of around 10mph will have that effect against each other.

Anyone with the relevant Physics/Aeronautics knowledge please enlighten us.

Re: attraction - JR
Then again... IMHO,

It could be a driver tendency to drift in the direction you are looking in. My instructor (Driving School RAF St Athen 1973) used to warn us of this and demonstrated it by making us drive along a white line (those wide concrete runways are useful for this... Don't try it at Heathrow), and getting us to look right/left...
You will generally drift to the direction you are looking in unless you are aware of this phenomenon.

John R
Re: attraction - John S
Yes, it's the Bernoulli effect. The same effect which is classically used to describe how a wing produces lift.

As air is forced between two cars running parallel, (or water is forced between two ships) its speed increases, its pressure drops. This is because (ignoring loses) total energy remains constant.

Vehicle mass is not a factor, and differential speed between vehicles isn't that significant (1). It's the velocity change betwen the air in the gap and the vehicle speeds that counts - generally the larger and closer the vehicles the more significant the effect.

(1) The effect of vehicles passing stationary cars or humans in the hard shoulder are more to do with wake turbulence than the Bernoulli effect. Hence vehicle size is significant in this case


Re: attraction - Alwyn
John S,

Sorry, I see you had already mentioned Bernoulli.

As you will know but others may not, we can demonstrate the Bernoulli Principle by taking a strip of paper say 8 inches long and 1 inch wide.

We then hold this against our chin just below our mouth so that the paper hangs down over our hand in front of our lips.

If we then blow over the upper surface of the paper it will surely rise, showing that the higher speed air above the paper air above is at a lower pressure than that below.

Wot a terrible description!!!!!!!!!
Re: attraction - John S

No, fine description! Always amazes people.

I've used a similar trick to teach kids about Bernoulli

Get a sheet of A4 paper, fold down an 25mm strip on each side, and rest it on the table with the edges supporting the centre above the table so it forms a tunnel. Then ask them what will happen when you blow gently under the tunnel.


Re: attraction - Darcy Kitchin
Also try with a spoon. Hold handle lightly between thumb and finger, let the spoon hang down and move into a stream of water eg from the kitchen tap.
Re: attraction - THe Growler
There is something I believe called target fixation where you do indeed steer involuntarily toward what you are looking at. I recall once in the US rounding a a curve too fast on a Harley and swinging out toofar towards the center and a pickup coming the other way. I was so intent on watching the pickup didn't clip me as he passed that the bike seemed to have a life of its own and to steer straight into the pickup. It semed to need brute force to re-establish the right line, where normally flicking the bike deeper into the corner and opening it up should have done it. A nasty moment which I learnt from.

An aviator friend of mine called this "target fixation", your movements involuntazrily follow what your eyes are focussed on.
Target fixation. - David W
Spot on Growler.

When at sea in a boat you might be looking for a marker buoy in a quarter mile wide channel out of a huge expanse of water, the idea being to pass either side of the marker within a hundred yards or so.

Actually what happens is the boat appears to be drawn to the buoy and you skim past with a near miss.....or not.

Same effect.

Re: Target fixation. - ian (cape town)
I recall reading somewhere that drunken drivers have lots of collisions with stationary police cars, as in their inebriated state they 'focus' on the flashing lights...
Re: attraction - alvin booth
A Spirax Sarco TD (thermodynamic) steam trap, a device which will allow condense to pass but not steam works on the Bernoulli theory.
The only moving part is a circular disk which seats on the inlet.
Many fitters have looked at this and still can't understand how it can work.
Re: Target fixation. - Ronnie Courtney
John S, Growler, and David - Go to the top of the class for your great explanations and examples!

Alwyn - the*non* scientific name you were thinking of that we simple sailors used to use (since we couldn't spell Bernoulli) for the phenomenon you rightly described relative to Replenishment at Sea (RAS) is probably "canal effect". This is so named since when a vessel moves forward, the water in a canal has to pass from ahead of the vessel to astern,and below the keel, so water on either each side and below the keel effectively "moves" in the opposite direction to the vessel herself. Bernooly, sorry, Bernoulli tells us that when a liquid speeds up, its pressure drops, so the water surface drops on either side of the vessel, and below the keel.

"Canal effect" is particularly evident in the Corinth Canal in southern Greece, where the combination of the nearly sheer sides of the canal and nothing under the keel result in quite large ships passing through with barely feet on either side, and even less under the keel than usual due to the sitting lower in the water, or "squat" effect, created by the forward motion, a major worry if your ship sees the ship ahead accidentally knocking great chunks of rock out of the sides! A bit like Growler in Manila traffic?

"Canal effect" is also very evident in the open sea during RAS, when the water passing between two ships is moving like a rushing torrent, quite visibly faster than their speed through the water, and is particularly critical when a large supply ship or tanker has a smaller ship on either side, with all the paraphernalia for replenishment binding them all together. It's not perhaps surprising that smaller ships, when breaking off from RAS, do so at high speed and under lots of helm in order to minimise the risk of collision. Less seriously, the lads (accidentally on purpose ...) occasionally used to walk back on the jackstay (or high wire) during personnel transfers so that a departing, possibly less than popular, officer or senior rating, got his feet (or sometimes more!) wet during his trip to the other ship.

Taking John S's point about turbulence when a large vehicle passes a pedestrian, I vividly recall, as a small boy , stepping confidently forward on the platform at our small local station in southern Scotland to board our usual stopping train, only to discover too late as I stood rooted to the ground that it was not stopping, but the Royal Scot, running in those days non stop from Euston to Glasgow Central, and very late, having built up speed all the way down the steep gradient from Beattock summit at one hell of a lick to make up time. No warning lines on the platforms in those days, and never mind whether it was Bernoulli's effect or turbulence, but I certainly learned a sharp lesson, and all about *Doppler* effect as the driver urgently sounded his whistle as the fourteen coach train thundered through the station just inches away, and I (and probably the driver, to whom I belatedly apologise) was left suffering from a very sharp attack of what I'll just call "Adrenaline effect"!

Regards and thanks to all concerned for a very interesting post, which you will see certainly jogged my memory - but not too much I hope .

Re: Target fixation. - BrianT

Bernoulli, target fixation, RDH???Doh!.

My guess is crap driving, but I could be wrong!
Re: Target fixation. - David W

It could be crap driving or the well known wander effect, ie before the vehicle steering systems are properly run in with a few miles on the clock. Mostly I find a car feels best at about 150,000 assuming decent maintenance.


Value my car