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A curious attraction.

  1. Apr 27, 2004 #1
    For about as long as I can remember, I have noticed that when I am standing somewhere while people are passing close by me I always feel a small "tug" as they go by. The little tug is always toward them. It always makes me wonder what causes it. After I feel the tug, about a second later I feel the air blowing against me as they pass.

    I have noticed the same effect on my vehicle on the road as I drive. A few days ago, as I was driving on a two lane county road, I had to stop for several minutes while a road crew was making repairs on the road. While I waited, several cars in the other lane was allowed to pass by my vehicle. The other drivers seemed to be in a hurry and had accelerated to about 40 mph as they passed me in the left lane.

    As they passed, I estimated that my minivan leaned over about an inch to the left as they went by. I decided to try a little experiment. I rolled down the window and held my hand up so I could feel the wind as they passed. Even though my van leaned over toward their vehicle, I did not feel any wind blowing against my hand until about a full second after my van leaned toward their vehicle.

    Then, when I felt the wind on my hand, it was blowing toward my van, yet, my van was never pushed away from them with any noticeable force. In other words, the pull was always the greatest toward their vehicle and not away.

    So, from my simple experiment, I am inclined to believe that the "tug" is not from the wind. If it was from a vacuum of air, I should have felt it on my hand.
    The only explanation I can come up with is it is from an electrostatic charge that builds up in the air as the other vehicle accelerates, making my van lean toward their vehicle from the attraction. Like bits of paper to a comb.

    I am quite sure that you have noticed this "tugging" effect on the road, or even when other people walk past you. I suppose that most people have just assumed that it is just from the movement of air. Maybe so.

    What do you think about it?

    I am not looking for an "armchair" theory please. I am looking for an answer from someone that has noticed this effect and really thought about it. Or from someone that knows the textbook answer. Thank you in advance for your answer! :smile:
     
  2. jcsd
  3. Apr 27, 2004 #2

    Rog

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    what you have to remember is the difference in pressure between the whole in the air behind the car that passes you and the pressure on the other side of your van. You dont say whether you opened both windows on both sides of the car, if you did then you should feel the air move through your cabin tending towards the low pressure area created by the passing car. The reason you dont feel you car shoot back with any particular force is that you must take into account the fact the you have 1 atmospheric pressure still pushing against the offside of the vehicle which would slow you movement back to a resting state. The most likely reason you didnt feel any wind on your hand as the car travels towards you is due to the aerodynamics of the car and the fact that the whole it creates in the air behind it is far more turbulent than the pressure wave that builds up in front.
     
  4. Apr 27, 2004 #3

    HallsofIvy

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    Personally, I only feel the "tug" when an attractive girl passes near.
     
  5. Apr 27, 2004 #4
    That is very funny HallsofIvy. I didn't know you had a sense of humor. I should have seen that one coming though. LOL


    After thinking about what Rog said about "the low pressure area created by the passing car", I thought of an experiment to do today, and I think that I may have found the answer to my question.

    First, I filled the kitchen sink with water. Then I took a coffee cup by the handle and pushed it around under the water with the bottom of the cup simulating the nose of the moving vehicles. I began to notice swirls of water on each side of the cup.

    So, whenever you have swirls, you also have vortexes and vortexes create low pressure areas.

    And that is the reason why my van is pulled toward the moving cars instead of being pushed away. I always thought the air on each side of a moving vehicle was like a v shaped wave pushing everything aside in its wake. Now I know that can't be.

    Even when we walk, we create an area of low pressure on each side of us because of swirling vortexes.
     
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