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How smoke travals at 70mph or dose it?

  1. Oct 28, 2009 #1
    how smoke travals at 70mph or dose it??????

    i have a ? that has been nagging at the back of my head for some time something i noticed for example a car traveling 70mph down a interstate all windows rolled up the passenger is smokeing he blows the smoke out of his mouth and the smoke floats freely inside the car nothing is forceing it to move its in no contact with the car yet it is keeping up with a car moveing at 70 mph how can this be i can be in room smokeing blow smoke out walk away it dose not keep up becuase nothing is forceing it to move im sitting in a car i hit the gas i feel my self go back in the seat the car is makeing me move but nothing is makeing the smoke move 70mph
  2. jcsd
  3. Oct 28, 2009 #2


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    Staff: Mentor

    Re: how smoke travals at 70mph or dose it??????

    The car is already moving at 70mph, so there would need to be something forcing the smoke to stop. That's Newton's first law.

    You said you feel the car accelerate - well the smoke filled air does too!
  4. Oct 28, 2009 #3
    Re: how smoke travals at 70mph or dose it??????

    i guess what im trying to ask is lets say the car is stoped smoke is floating freely imside amd out side of the car i pull of the smoke out side stays and the smoke inside moves with me how can this be the car is makeing it self move and me but not the smoke
  5. Oct 28, 2009 #4
    Re: how smoke travals at 70mph or dose it??????

    the car is filled with air and smoke, and the air and smoke move with the car as it pulls off.
    for the smoke to stay put as you pulled off (ref. to the ground outside), and effectively move towards the rear of the car it would have to displace all the air in the cabin which is causing an obstruction between itself and the back of the car.

    are you comfortable that the air inside the car moves with the car?
    there would be some movement of the air, but it wouldnt *all* suddenly get pressed against the back windscreen, because that would cause a vacuum towards the front of the car (which would 'pull' the air forwards)
    If you want it explained mathematically then somebody else will have to help you there, but a lot of this is down to air pressure I believe
  6. Oct 28, 2009 #5
    Re: how smoke travals at 70mph or dose it??????

    Think of the smoke as an opaque form of air- all the air in a car doesn't suddenly rush to the back when you accelerate creating a vacuum in the front seat.
  7. Oct 28, 2009 #6


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    Homework Helper

    Re: how smoke travals at 70mph or dose it??????

    During acceleration, there's a net forwards force exerted on the air by the interior of the car. The pressure at the front of the interior is slightly less than the pressure at the rear of the interior. The net pressure differential times the effective area represents a force that in this case will be equal to the mass of the air times the rate of acceleration of that air.
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