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Thread: Air particles and acceleartion

  1. #1 Air particles and acceleartion 
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    Jul 2010
    This May seem pretty straight forward. But me and my friends are unsure and it's really bugging us. We like science, but when it comes to knowledge and understanding we have very little.

    This is the scenario.

    A car is travelling along a road. As it hits the air it slows down. We know this is because if air resistance. We also know that the air will slide around the car and the bettter it does so the better aerodynamics it has. Now this is the burning question. At one point on the car an air particle will hit it directly on a centre point so that it cannot go over or slide around the car as such. So what happpens to this air?

    I know the energy from the car is transferred into the air and is lost in other ways too noise, friction, heat etc.

    But it's just that one point where it is directly centred to the car.

    We have two theories. One is that te air will rebound off and intront of the car directly forward.

    Another is that it will stay in contact with the car and never be pushed forward but will slide across the car when the particle is exerted to some other force or another air particle rebounds of it.

    Fir arguments sake the car is travelling at a constant speed.

    The reason this all came about us that if you move your hand toward a solid flat surface, constantly accelerating, you will never feel the air hit the wall and rebound back on to your hand because your hand is always going to be moving faster or atleast the same speed than the air you are accelerating by pushing it.

    Sorry if this all sounds confusing, like I said I am really a science novice, although it does really interest me. Some areas more than others. E.g space and astronomy and other phenomenan associated with it.

    Thank you if anyone can help

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  3. #2  
    Forum Bachelors Degree x(x-y)'s Avatar
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    Jul 2010
    Well this really depends upon the wind speed and direction. If there is an overall resultant force of the wind acting upon that 'direct hit air particle' then it will move in that direction corresponding to the wind speed. There will never be a 0m/s wind speed and so that air particle will always move.

    But, for argument's sake, let's say there is no wind. That particle will hit the car directly and be rebounded of the front of the car due to momentum (p=mv), it will then be carried away by one of the airflows around the front of the car- which depends upon the angle of deflection and the speed of airflows on either side.

    Hope this helped you, I'm not sure if it's 100% correct, but I haven't studied this type of physics for a while, so...

    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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  4. #3  
    Moderator Moderator Janus's Avatar
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    Jun 2007
    The thing to keep in mind is that air molecules, even with no wind are not static. They are in constant random motion, jostling each other. The average velocity of any given molecule would be several hundred meters/sec. (It is the resultant force of all these air molecule traveling at high speeds that causes what we call air pressure.)

    So the chances of any given air molecule striking the exact center point of the car while its direction of motion is exactly opposite that of the car is slim to remote. Even if it did, it would rebound directly backward and very quickly be deflected to the side by some other randomly moving air molecule.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone

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