Acceleration v Constant Velocity

  1. This question might seem strange or dumb.. At least it does in my mind, however I phrase it..
    Anyway, I'm wondering about the reasons for acceleration being felt and constant velocity not being felt by a human body or other object. If you're sitting in a train or a car and it accelerates rapidly to 100 mph, a bottle might tip over, whereas if you're moving at constant velocity 100 mph, it won't. What exactly is the process by which the energy of that acceleration is felt by any object inside the car or train?

    I should note that I don't study physics of any kind, so I won't be familiar with any mathematical formulae that might explain this!
  2. jcsd
  3. Inertia is that "process" (it's not really a process but a fundamental principle of nature). Inertia is the ressistance of objects to some change in their state of motion. That is, if an object is still, it tendes to remain still and if it is moving with constant (in magnitude and direction) velocity, it tends to keep that velocity stable. When you see the bottle going backwards when the train accelerates, that happens because the bottle was moving with a constant velocity which (due to inertial) tends to keep, so when the train accelerates the bottle stays behind. The same principle applies to every object with mass.
  4. mfb

    Staff: Mentor

    As far as we know, physics is the same in all reference frames - there is no "absolute velocity". If you don't look out of the window, you cannot measure the speed relative to the ground (neglecting issues with wheels, sound from the motor and so on).
    This is a fundemental concept in the theory of special relativity.

    There are several other motions which would be relevant otherwise:
    - earth rotates around its center, the surface moves with ~300m/s (depending on your latitude) relative to its center
    - earth rotates around sun with ~30km/s
    - the solar system rotates around the galactic center with ~200km/s
    The train is slow compared to that.
  5. So until an amount of energy equal to the object's inertia has influenced that object, it's initial position will be maintained?
    For example, if the table on which the bottle is positioned in the train were entirely frictionless, the bottle's position would be completely fixed in space and the table would move forward independent of the bottle?
    Or in other words, the reason a bottle tips over in the opposite direction of the train's motion is that the friction between the bottom of the bottle and the table means the lower part of the bottle hangs on to the table while the top half's position wouldn't move if not because it's obviously attached to the bottom half.
    Not sure I'm properly communicating what I mean, but hopefully you'll understand..
  6. mfb

    Staff: Mentor

    That question does not make sense.

    It would be fixed relative to the initial motion (for example "fixed relative to the ground"). There is no "fixed in space".

  7. About the first quote - I'm probably conceptualizing inertia in an entirely illogical way, but I see it as an object having some quantifiable measure of inertia, or a kind of "negative energy", so the object won't move until it's been influenced by an amount of energy equal to that deficit.. does it make sense? Or am I way off?
    Right, I wasn't sure how to phrase the 2nd quote, but I mean fixed at some coordinate, or as you say, fixed in relation to the ground.

    EDIT: I know there's no such thing as "negative energy", but I'm unsure how best to describe what I mean!
  8. mfb

    Staff: Mentor

    No, that is wrong.
    Any force will accelerate an object, unless there is a corresponding force of same strength in the opposite direction - friction often provides this.
  9. Ah ok, I'll go read about inertia on wiki. Thanks for your time!
  10. An object indeed has a quantifiable amount of inertia. It's directly tied into its mass. Twice the mass gives you twice the inertia. In other words, twice the mass is twice as hard to accelerate.

    In order to accelerate an object, you also indeed have to input energy (mechanical energy) into it. You do this by applying a force. However, there is no minimum force or mechanical energy required to accelerate an object. ANY force or mechanical energy will do it. It's just that the more inertia an object has, the less it accelerates...but it accelerates nontheless.
  11. russ_watters

    Staff: Mentor

    Someone really should say:

  12. Fredster....
    so if you combine post 9 and 10, you'll see the smallest force [f] will accelerate even
    a huge mass [m].....but only the smallest amount......but if friction is involved, you'll have to apply enough force to
    overcome that friction. absent friction, a mass will continue to follow a geodesic path....
    like a straight line if there is no gravity......or a curved path if gravity is present......
  13. Thanks for your replies. I'd say all my questions have been answered, even the ones I was pondering and hadn't posed yet!
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