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Newtons Law

  1. Jan 12, 2005 #1
    Not sure exactly:
    1. Why do passengers jerk forward when the car suddenly stops? I know it is somethin' relative to Newton's law but I don't know which law it is and how it actually happens.
    and
    2. Why do the passengers feel like they are pushed to the outside of a turn.
     
  2. jcsd
  3. Jan 12, 2005 #2
    1.) try to see it the other way.. you are not jerk forwand.. is your car jerk backward.. this backward motion makes you FEEL you are moving forward.. but actually want happening is the other way around...
    2.) same thing..
     
  4. Jan 12, 2005 #3

    dextercioby

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    Both questions have the same answer:because they experience (are acted by) inertial forces.These forces are typical of/specific to noniertial reference frames.

    Daniel.

    EDIT:In the second example,the force has a name:centrifugal inertial force.
     
  5. Jan 12, 2005 #4
    hey dex.. don't use so many technico terms in a question like that... do you think someone who dont understand this simple effect will understand your
    ???
     
  6. Jan 12, 2005 #5

    dextercioby

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    Well,Vincent,didn't u have enough time to realize that i tend to give people as rigurous as possible answers ?If i don't,and if i give them scientifically unrigurous explanations,they might buy them without checking other sourse for veridicity.So putting "fancy"/"technical" terms into explanations is not some weird unexplainable habit of mine.


    Daniel.

    PS.Would u rather have been given an incorrect,yet comprehendable explanation,or a correct and uncomprehendable one?? :wink:
     
  7. Jan 12, 2005 #6
    I'm not quite sure what are are saying here. you say that you don't jerk forward and it's that the car that jerks backwards that is why you feel this force? Also, is this Newton's first law?
     
  8. Jan 12, 2005 #7

    dextercioby

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    You definitely jerk forward,u feel it and somebody at rest on the groud sees it as well.However,keep in mind that when a car is decelerating,its velocity vector (and the coordinate/position vector) is pointing forward (in the direction of movement),but the acceleration vector is pointing backwards (that's the definition of deceleration:negative acceleration due to opposite sense of the vector acceleration wrt to the position and velocity vectors) which means that the car (and you with it) experience a force pointing backwards.Here steps in the first principle of Newton which tells us that bodies tend to keep their motion tendency,viz.when they are rest,they tend to remain at rest,when moving in direction,they tend to move in that direction,in the case when external forces would alter their movement.In the case of the car,your tendency is to go forward (together with the car,of course),yet an external force (the one from the engine of the car) will alter your movement,and here the first principle comes in and says that an inertial force will act upon your body in the direction of movement,exactly to compensate the effect of the external force.It's acceleration is exactly the same to the one the external force is giving,but,obviously,the sense is opposite.


    Daniel.
     
  9. Jan 13, 2005 #8
    Is this anything to do with relativity?...
     
  10. Jan 13, 2005 #9

    DaveC426913

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    It is Newton's 1st Law: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

    The passengers do not jerk forward. They continue along the same path they were following (at 60mph) - independent of the car's motion - while the car stops underneath them. The feeling of a force pushing them is merely a subjective illusion caused by the car moving against their feet and bums as it slows underneath them.


    For exactly the same reason. The passengers are not pushed to the outside of the car. They continue along the same path they were following - independent of the car's motion - while the car tries to turn underneath them. The feeling of a force pushing them is merely a subjective illusion.
     
  11. Jan 13, 2005 #10

    DaveC426913

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    No, nothing.
     
  12. Jan 13, 2005 #11
    It has something to do with Gallelian principles of relativity, but nothing to do with special or general relativity.

    In this scenario, you have 2 relative reference frames, the moving car and the ground. As the car stops and returns to the earth's frame of reference, you continue to move forwards with the car's original reference frame, until the seatbelt catches you and pulls you back to the stationary reference frame.
     
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