Many times I was sitting in a train, watching closely nearby people.

If the vehicle changes it's movement rapidly (brakes, turns, etc), then obviously, people feel the inertial pseudo-force that might even make them fall. They don't fall, however, since by the power of their muscles they keep their bodies still, and the friction keeps them in place.

All of the above, however, can't affect the movement of one's hair or clothing. I mean - this inertial pseudo-force affects both one's head and hair, doesn't it? Let's assume one is sitting in a train that brakes forcibly, and they're looking in the direction of the movement of the vehicle. They have muscles in their neck, and so they can stop their head from moving and ramming the backrest of a seat ahead; but, they don't have any muscles in their hair! So, if they forcibly make their heads still, their hair should move forward with respect to the head, just as if under the effect of the wind, shouldn't it? (with the wind imitating the inertial force) Same applies to clothing, etc.

I've been watching very closely - no such thing happens. Why...? I've been wondering for some time, and I can't come to an answer.

  • $\begingroup$ Maybe your trains have operators who brake and accelerate very smoothly... $\endgroup$
    – DJohnM
    Mar 31 '15 at 23:34
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    $\begingroup$ @User58220 Definitely not. $\endgroup$
    – gaazkam
    Apr 2 '15 at 11:53
  • 3
    $\begingroup$ Use should have long hair, then you will notice the movement of hair, although it won't come out as the hair-skull bonding is strong enough to withstand that much force. $\endgroup$ Oct 1 '15 at 17:37
  • 5
    $\begingroup$ I see this happening all the time. The bus stops suddenly, loose clothing, necklaces, the fabric handles on the top bar in the bus, etc. All sway forwards. I don't know why you don't see it happening (do you live somewhere where people wear unusually tight clothes?) but I assure you it happens just as you'd expect $\endgroup$
    – Jim
    Oct 1 '15 at 19:20

If a vehicle (bus, train) rapidly stops (or accelerates, or turns), why don't our hair or clothes move from the inertial force?

The short answer is that clothes move and you can quickly perform an experiment.

Take a metro train, preferably at a time when it is empty, and put a long scarf (the heavier the better) on one of the horizontal bars people grip to maintain their equilibrium when standing.

Take a sit and watch the scarf as the metro takes curves, accelerates when it leaves stations or slows down while entering stations.

You will see that the scarf moves, it is true, not in a spectacular way but the phenomenon is observable.


It happens, it's just (as Sidd said) not as noticeable. I would like to suggest the following experiment: wear a chain, necklace, or bracelet--something with some dangle to it. You should see that during braking/acceleration it no longer hangs straight down.

  • 1
    $\begingroup$ This video, youtube.com/watch?v=s4tuvOer_GI shows Colonel Stapp enduring high g's and hair flying... $\endgroup$
    – DJohnM
    Mar 31 '15 at 23:48
  • $\begingroup$ A great idea with the experiment, however, there are problems: I can't make my body completely still (regarding to the vehicle) during hard braking... And great video, thanks. $\endgroup$
    – gaazkam
    Apr 2 '15 at 12:01
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    $\begingroup$ Hm. How about next time you're in a car hanging something from the rear view mirror? $\endgroup$
    – zeldredge
    Apr 2 '15 at 12:17

The inertial force is proportional to both the acceleration of the system and the mass of the object in question. Hair are light, and thus you don't see the effects without severe acceleration. The clothing are worn, and hence supported by one's body. So they won't move till the body does.

  • $\begingroup$ Even the tightiest wind makes hair move. Severe braking doesn't seem so... $\endgroup$
    – gaazkam
    Mar 31 '15 at 21:39
  • $\begingroup$ For an advanced experiment: place a bag or backpack on the floor of the train car and tie a helium balloon on a string to the bag... $\endgroup$
    – DJohnM
    Mar 31 '15 at 23:35
  • $\begingroup$ Wind affects only the hair. Braking affects all of the person, so the net difference (between body and hair) is smaller. $\endgroup$
    – Jiminion
    Apr 1 '15 at 12:56
  • $\begingroup$ Inertial force is proportional to the mass of the object in question, right. However, the acceleration of the object in question is not proportional to it's mass; rather, the module of the acceleration of the object in question is equal to the module of the acceleration of the system. At least if I remember correctly my high school physics lessons... But if I'm right, then the hair movement should definitely be visible? $\endgroup$
    – gaazkam
    Apr 2 '15 at 12:05

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