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Why are we thrown backwards when a train stops?

  1. May 24, 2012 #1
    When it stops, you move forward but at the very last moment just before it comes to a complete halt (when it sounds relatively quiet), it moves back. On a bus or a car, I'm guessing that would be caused by the front of the car 'jerking' back up, but a train seems more...rigid? Also, why do the front of cars lower when they decelerate?
     
  2. jcsd
  3. May 24, 2012 #2

    tiny-tim

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    hi autodidude! :smile:
    i think it happens if the driver releases the brake just before the vehicle stops, so the deceleration is less
    if you hang something from the roof of the car, it will hang at an angle forwards

    this shows the direction of the "fictitious gravity" (ie gravity plus the fictitious force -ma) that exists in the decelerating frame of reference of the car

    the weight effectively acts in the same direction, starting at the centre of mass and therefore going closer to the front than usual …

    so there's more weight over the front wheels, and less over the rear :wink:
     
  4. May 24, 2012 #3

    Ken G

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    Another effect to bear in mind is that your body is bracing against the forward fictitious force you are feeling. You don't know when that force will go away, so you continue to brace until after you notice the force is gone. So the backward thrust you feel could come partly from you-- not just from the de-flexing of the vehicle when the cause of strain is removed.
     
  5. May 24, 2012 #4
    While the train is stopping there is a certain deceleration that you feel. A the instant the train stops, the decelaration goes from that value to 0 (zero) so you feel a sense of a jerk.

    Suppose you were sitting facing backwards as the train is stopping. You would be pressed into the back of the seat, which being a little springy will let you sink into it a bit. At the moment the train has stopped you will feel no accelaration ( deceleration) and you will no longer be pushing into the seat. With no acceleration the spriginess of the seat will now push you back out, or in a backwards sense as you say.

    Both tt and kg have given applicable answers and I thus think it is a combination of factors at play here.
     
  6. May 24, 2012 #5

    rcgldr

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    Because their center of mass is above the the pavement, createing a torque due to the pavement exerting a backwards force at the contact patch of the tires and the forwards reaction force due to deceleration of the car at the center of mass.

    The brakes themselves also generate a downwards torque onto a car during deceleration.
     
  7. May 24, 2012 #6
    Re: Why are we thrown back-wards when a train stops?

    The force of friction which is slowing the car is acting at the road/tire interface and its vector points rearward. The center of mass of the car is higher then road surface. Since the force is not directed through the center of mass there is a torque on the car which tries to roll it forward.

    This tendency is usually lessened through the engineering of the brakes and suspension. As you apply the brakes the rotor tries to pull the caliper along. On the front end the force on the caliper is transfered to the suspension in such a way that the front is pushed up, on the rear they reverse it so the rear end gets pulled down. It is actually possible to design a suspension that would nose up when braking, but it would feel unnatural and awkward so nobody does that.
     
  8. May 28, 2012 #7
    Thanks a lot guys, very helpful
     
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