Can you get pulled under a train?

[dextercioby]

TL;DR

Practical issue when staying close to train tracks. Can/Should you do it? What if the Hanoi train was travelling at 100 km/h?

Assume that a 100m long (probably irrelevant) train travelling at 30m/s, with a mass of 100.000 Kg passes next to a man/woman of 100 Kg standing still at 1 m away from the track. Since air is massively displaced by the train, creating like a small vacuum/low pressure area next to its surface, I reckon a certain pull may be applied to anything the train passes by, so in this case a person. My question is very simple: this pull can be quantified by a force. Is this force enough to pull the person under the train? What if the distance was 50 cm?

 

[.Scott]

If that 100Kg person is not caught off guard, he/she should be fine.

I have been next to high-speed trains before. And I have also done some parachute jumps from planes with no door and where I needed to climb out to the midpoint of the wing before letting go.

I also do a two-hour walk every day and have had some large vehicles (like Camping RVs) cross by at about 65mph just inches from me. Clearly there is a gust, but I sensed no pull from them at all.

I could imagine someone being surprised and toppled by the breeze - perhaps into the train.
But sucked under the train is really not in the cards - not for someone at 100Kg.

At the 58 second mark in this video, you can catch the effect a high speed train has on the grass nearby.
If you hunt around, you should be able to find better examples of grass or litter being moved about by passing high-speed trains.


Clearly standing near the tracks is not a great idea - but for other reasons. First, you may not correctly estimate how wide the train is (or what projections there may be) before it gets too close to move away. Second, looking at some videos, I notice that in a some cases the high-speed trains can kick up quite a bit of dust. So, if standing near a moving train wasn't exciting enough for you, you could end up standing there blinded with sand in you eyes.

 

[renormalize]

Is this force enough to pull the person under the train?

Probably about as likely as being pulled under by a supernatural Hollywood force:

 

[erobz]

TL;DR Summary: Practical issue when staying close to train tracks. Can/Should you do it? What if the Hanoi train was travelling at 100 km/h?

Assume that a 100m long (probably irrelevant) train travelling at 30m/s, with a mass of 100.000 Kg passes next to a man/woman of 100 Kg standing still at 1 m away from the track. Since air is massively displaced by the train, creating like a small vacuum/low pressure area next to its surface, I reckon a certain pull may be applied to anything the train passes by, so in this case a person. My question is very simple: this pull can be quantified by a force. Is this force enough to pull the person under the train? What if the distance was 50 cm?

I don’t know how to calculate it, but I believe you are looking for the boundary layer pressure.

 

[Baluncore]

I would expect the strongest pull from the platform towards the tracks, would be by the lower pressure that immediately follows the train.

 

[jack action]

Myth ... busted.

 

[BWV]

Maybe by this one

 

[DaveC426913]

How rigidly do you mean "pulled"?

Any train moving at any appreciable speed (including a subway car) will have a bow shock and rebound that can nudge a nearby person off-balance. That is potentially enough to have them fall - or merely stumble - the wrong way and end up on/in/under the train. Does that count?

If you want to be rigid about literally getting "pulled" under a train, then you'd have to start defining how well set they are, stance, traction, area, mass, etc.

So, I'd say the answer is: yes, it can happen. Until and unless you define your parameters better.

 

[jrmichler]

TL;DR Summary: Practical issue when staying close to train tracks. Can/Should you do it? What if the Hanoi train was travelling at 100 km/h?

My question is very simple: this pull can be quantified by a force. Is this force enough to pull the person under the train?

The magnitude of the aerodynamic effects of a vehicle are mostly due to the frontal area. The frontal area of a Class 8 truck (semitruck) is about the same as that of a train. I had a lot of experience hitchhiking back in the 1970's, and experienced many semis going past at speeds over 70 MPH (110 km/h). The flat front rigs back then moved a lot of air, especially with a cross wind. The bow waves from the trucks were enough to knock me sideways 2 to 3 feet on a windy day. That was with my legs braced and locked. I skidded on the gravel shoulder of the road. Modern aero trucks have nearly eliminated the bow wave. I would expect the aerodynamic effect of a train to be very similar to the effect of a semitruck.

I was never sucked toward the truck, regardless of the wind direction.

Any train moving at any appreciable speed (inlcduing a subway car) will have a bow shock and rebound that can nudge a nearby person off-balance. That is potentially enough to have them fall - or merely stumble - the wrong way and end up on/in/under the train.

Definitely a possibility, especially if a person is not paying attention.

 

[Rive]

Any train moving at any appreciable speed (including a subway car) will have a bow shock and rebound that can nudge a nearby person off-balance. That is potentially enough to have them fall - or merely stumble - the wrong way and end up on/in/under the train.

Closest to that scenario is about my biking experiences on high traffic roads. But the catch in that is, that on bikes you automatically compensate to keep balance - and that's without much conscious thoughts about possible consequences.

 

[sophiecentaur]

And I have also done some parachute jumps from planes with no door and where I needed to climb out to the midpoint of the wing before letting go.

I did a jump from a De Havilland Rapide (BiPlane) once (my first jump) and your comment reminded me. It felt like I was in a 1930s film and was probably the best bit of the whole process with the twin engines chugging along like bus engines. Sitting on the floor with the door open was novel - nothing to secure us and I guess we could have been tipped out(?).

 

[DaveC426913]

< digression >

I did a jump from a De Havilland Rapide (BiPlane) once

This is one of the most beautiful planes of its era or most eras. I am envious.

I live very near the Hamilton Warplane Heritage Museum, where you can buy a flight in a range of vintage aircraft. While it would be awesome to ride in a PBY Catalina/Canso ($800) or a Lanc ($4300!),

I think the best bang for my buck might be an open-air biplane like their Boeing Stearman ($500):

< /digression >

 

[.Scott]

I did a jump from a De Havilland Rapide (BiPlane) once (my first jump) and your comment reminded me. It felt like I was in a 1930s film and was probably the best bit of the whole process with the twin engines chugging along like bus engines. Sitting on the floor with the door open was novel - nothing to secure us and I guess we could have been tipped out(?).

My 6 jumps were from a Cessna 182. I was more concerned with the reserve parachute opening before I got out of the plane. I was with 5 other jumpers, we left one at a time. As the lightest of the jumpers, I was always last to jump and needed to do a lot of shuffling around before then - all the time protecting the handle to the reserve chute ripcord.
Had that cord been pulled while I was still in the plane, the results may not have been as pleasant as being pulled under a train (my nod to the OP). That doorless doorway was plenty large and created a breeze that would likely have caught the released chute and pulled it outside. At that point, the 100+mph air stream would taken over and attempt to pull me through the wall of the plane. Several scenarios are possible from there - none of them good.

 

[jsurow]

It seems that this could be modeled by the Bernoulli equation:
P + (1/2)ρv² + ρgh = constant
where:

P is the static pressure of the fluid.

• ρ is the density of the fluid.
• v is the velocity of the fluid.
• g is the acceleration due to gravity.
• h is the elevation or height of the fluid.

That is, the train acts like a fluid moving through the air surrounding it, and it will cause a lower pressure perpendicular to its axis of travel. That negative pressure would tend to pull the person standing near the train toward the train. With appropriate assumptions of the above variables, the inward force on the person could be estimated. And as another person suggested, we probably need to compute the boundary layer effect to figure what the velocity of the fluid(air) is.

Just my guess....

 

[DaveC426913]

I should say it is more pronounced in the enclosed volume of an underground subway station.
A train coming into the station at 30mph, when you are five feet away from the tunnel exit can pack a bit of a wallop.

 

[sbrothy]

I did a jump from a De Havilland Rapide (BiPlane) once (my first jump) and your comment reminded me. It felt like I was in a 1930s film and was probably the best bit of the whole process with the twin engines chugging along like bus engines. Sitting on the floor with the door open was novel - nothing to secure us and I guess we could have been tipped out(?).

You seem to constantly reveal new surprising character depths! I'm impressed! And this has nothing to do with gender.

 

[tech99]

I have noticed the effect when sailing a little boat near a large tanker, which was anchored. As a fast tide was flowing between us, I felt myself strongly drawn towards the tanker due to the Bernoulli effect. Managed to get clear.

 

[256bits]

Large ships are weird.
I started reading up on this due to the post.

A few tug boats have collided with the larger tanker due to induced currents and hydrostatic effects from the larger ship. The water in front of the larger ship gas a greater pressure, lower pressure amidship, and larger pressure at stern.
In channels, the bow can be pushed away from the nearer shore while the stern will angle towards the nearer shore. Boats passing can collide due to interactions between the different pressures of the water between bow, midship, and stern. With a shallow channel, the ship can 'sink' into the water increasing its draft.
Ship-to-ship interaction can be up to a separation of 100's of meters. A critical distance is where, up to that point, the force has increased, and then begins decreasing. This effect, obviously against intuition ( where the force should decrease continuously with distance ), is attributed to wave action between ships being amplified by reflections.

 

[256bits]

It seems that this could be modeled by the Bernoulli equation:
P + (1/2)ρv² + ρgh = constant
where:

P is the static pressure of the fluid.

• ρ is the density of the fluid.
• v is the velocity of the fluid.
• g is the acceleration due to gravity.
• h is the elevation or height of the fluid.

That is, the train acts like a fluid moving through the air surrounding it, and it will cause a lower pressure perpendicular to its axis of travel. That negative pressure would tend to pull the person standing near the train toward the train. With appropriate assumptions of the above variables, the inward force on the person could be estimated. And as another person suggested, we probably need to compute the boundary layer effect to figure what the velocity of the fluid(air) is.

Just my guess....

I get,
If the air behind the person was still and in front at 100 mph, the force on the person would be around 5 pounds.

What do you get as a check?

 

[DaveC426913]

Large ships are weird.


With a shallow channel, the ship can 'sink' into the water increasing its draft.

If I understand correctly this was actually used as a necessity in some cases.

A ship's total (stationary) height exceeds the bridge clearance by a few metres. They must run the ship through at-speed, which drops its height by a sufficient amount to clear the bridge.

Sounds urban-myth-y, I know. I should prolly find a citation.

 

[sophiecentaur]

I felt myself strongly drawn towards the tanker due to the Bernoulli effect

HAha. Too much of a stressful situation to subject yourself and to study (in-depth; no pun intended) IMO. I have (also) seen a scaled down version on a narrow boat in a canal. Even at the permitted speed of 4mph, if you go close to a vertical sided bank, you can see the profile of the water level along the hull. Noticeably lower near the stern and you can 'play with' the tiller to see how that level changes with separation. If I was a duck . . . .

As the list of variables above shows, the equivalent of that in air will demand higher speeds for any serious risk. That line they paint near the edge of the platform in stations is more to stop idiots jockeying for a good position for boarding when the train stops. 100mph?? I'd be standing well back, whatever the physics tells us.
I'd imagine the greatest force would be at the tail end of the train.

Air pressure has a real effect when a small, under-powered 'box sided' van tries to overtake a large truck. There's a point where you just can't go past. That's got to correspond to more than just a few tens of N.

 

[.Scott]

Large ships are weird.

... and risky ...

While working on the helm system for the Ford, I was informed that the reliability of the steering system was most critical during underway replenishment operations. During that time, the aircraft carrier and the supply ship run parallel to each other, precisely matching speed and direction. From the wiki article:

Alongside connected replenishment is a risky operation, as two or three ships running side by side at speed must hold to precisely the same course and speed for a long period of time. Moreover, the hydrodynamics of two ships running close together cause a suction between them. A slight steering error on the part of one of the ships could cause a collision, or part the transfer lines and fuel hoses. At a speed of 12 knots, a 1-degree variation in heading will produce a lateral speed of around 20 feet per minute. For this reason, experienced and qualified helmsmen are required during the replenishment, and the crew on the bridge must give their undivided attention to the ship's course and speed. The risk is increased when a replenishment ship is servicing two ships at once.

That last statement does not apply to US Navy carriers - since they can only be supplied this way on their starboard side.