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Homework Help: Forces in train

  1. Oct 13, 2012 #1
    Why are we moving back when train starts accelerating.....can i get answers in terms of forces please?
  2. jcsd
  3. Oct 13, 2012 #2


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    Hi :smile:xx!

    It doesn't have anything to do with forces.

    It's because of the absence of forces …

    there's no force moving you forward, so when the train starts accelerating, you get left behind! :wink:
  4. Oct 13, 2012 #3


    "Newton's First Law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. It may be seen as a statement about inertia, that objects will remain in their state of motion unless a force acts to change the motion. Any change in motion involves an acceleration, and then Newton's Second Law applies; in fact, the First Law is just a special case of the Second Law for which the net external force is zero.

    Newton's First Law contains implications about the fundamental symmetry of the universe in that a state of motion in a straight line must be just as "natural" as being at rest. If an object is at rest in one frame of reference, it will appear to be moving in a straight line to an observer in a reference frame which is moving by the object. There is no way to say which reference frame is "special", so all constant velocity reference frames must be equivalent."

  5. Oct 14, 2012 #4
    hi tiny-tim,
    So their is absence of force which makes us move backward,and does resistance of floor prevent us to fall backwards?:D
  6. Oct 14, 2012 #5
    Video ,expanation and link was quite helpful.thanks alot!
  7. Oct 14, 2012 #6


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    hi :smile:xx!


    if the floor is slippery, then we move backwards

    if it isn't, then our feet have to move with the same acceleration as the train, but our centre of mass feels no reason to move at all, so we start rotating backwards (say anti-clockwise)

    the way to counter this is to lean forwards so that the (say) clockwise moment of our weight (relative to our feet) cancels out this anti-clockwise angular motion

    another way of looking at it is that, in our non-inertial (accelerating) frame, there is an effective gravity which is the vector sum of g and minus the acceleration, -a

    that effective gravity is at an angle, and we will topple unless the line from our centre of mass at that angle goes through our feet :wink:

    (same reason why a motorcyclist leans sideways going round a corner, except that his acceleration then is centripetal acceleration, sideways)​
    Last edited: Oct 14, 2012
  8. Oct 14, 2012 #7
    Allrite thanks alot bro.So now i get it why motorcyclists lean sideways completely,its because the centre of mass still wants to move in straight line but bike needs to ride round corner so it makes certain angle which provides centripetal force towards centre and also the friction of ground would help motorcyclist to turn round the corner!!
  9. Oct 14, 2012 #8


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    no, the centre of mass wants to fall, and the only thing that can stop it is the wheel being in the way; the bike needs to ride round a corner which creates centripetal force (in the non-inertial reference frame of the bike), which would make it fall if the bike was upright, so the rider leans at the exact angle that keeps the wheel in the way! :wink:

    (leaning doesn't make the bike turn … it can turn without leaning (that's what the steering wheel does!) … leaning only stops the bike from falling over!)
  10. Oct 14, 2012 #9
    I got some part of it but i still didn't get that how would wheel stop the centre of mass from falling?
  11. Oct 14, 2012 #10


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    if the bike is stationary, and the centre of mass is over the wheel (or rather, the line between the two wheels), it won't fall, will it? :smile:

    (but if it's not over the wheel, it will fall)
  12. Oct 14, 2012 #11
    Yes i completely get it now.thanks alotttt! ;)
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