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Basic F=ma question

  1. Oct 5, 2006 #1
    If f = ma, then in a case of a vehicle accelerating/decelerating why does there need to be a friction force?

    Say you had no friction force (I know this is not possible) and a car sitting on level ground, then apply a force from a high powered fan (Drag) to the front of the car. why does this not casue and acceleration backward?

    Is every force tied to earth? Say when I push something I apply force to the ground to be able to do that, but with air what is the air pushing against (gravity?)
     
  2. jcsd
  3. Oct 5, 2006 #2

    Hootenanny

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    Have you any reason to think that it doesn't accelerate?
     
  4. Oct 5, 2006 #3

    berkeman

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    Think of a sailboat and how it moves about....


    EDIT -- Well, at least how it moves about downwind, not the tacking upwind part.
     
  5. Oct 5, 2006 #4
    Think of it this another way -- what causes a puddle to evaporate?
     
  6. Oct 5, 2006 #5
    Well, that is my question.
    I've read in my Vehicle dynamics book this, as well as on the internet in several places.

    So if it does accelerate then how?

    What I think is it would but the tires wouldn't spin.
    But this is of course not a "real" situation, so the books just state this because you could never have zero friction.
     
  7. Oct 5, 2006 #6

    There is a friction between the boat and water though, same question if that friction wasn't there I would expect the boat to "glide" backwards. But I haven't read about boats, just cars and it's stated without friction, no acceleration. Hence the confusion.
     
  8. Oct 5, 2006 #7

    I appreciate the responses and say this with a smile.
    You must all be physics teachers, because I ask a question and get three questions in return.:tongue2:
    By the way, I'm not a student - I finished a masters degree in Computer Engineering 9 years ago, and I remember very little physics so I'm just trying to catch back up for the fun of it.
     
  9. Oct 5, 2006 #8

    Hootenanny

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    You have hit the proverbial nail on the head. The car would accelerate because there is a net external force in the direction of the wind. However, if you are talking about the car moving under its only propulsion on a surface where there is no friction, the tyres will have no traction and therefore the wheels will just spin and an external force will not be produced, thus the car will not accelerate.

    And to comment on your above post, I am also a student studying for my Master's in Mathematical Physics.
     
    Last edited: Oct 5, 2006
  10. Oct 5, 2006 #9

    berkeman

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    Okay, one last try, and hopefully this will apply more directly to your postulated situation. Consider a solar sail. It's up in space, so that meets your "no friction" criteria. The solar sail works because the momentum of the solar wind is partially transferred to the sail, which pushes the sail in the direction that the solar wind is flowing. The energy to cause that acceleration comes from the sun's original reactions.

    Same with an object on a frictionless surface on earth (like a puck on an air hockey table). The energy comes from whatever made the wind (like a fan or weather patterns), and some of the momentum of the moving air molecules is transferred to the gliding object to accelerate it.

    I don't know where you got the idea that there would be no acceleration. Can you post a web pointer to that assertion? The source must mean something different.
     
  11. Oct 9, 2006 #10
    In this case, the original question was if a fan was blowing from the front of a car, why wouldn't it accelerate w/o friction. Look at it this way -- the fan would move backward as air moved forward. You couldn't screw the fan down because friction is what holds the screws in the material. You couldn't turn the fan without friction becuase you couldn't keep a grip on the shaft. There is an endless string of arguements you could make to a point where friction would be needed in order for this system to work.

    In other words, as much as friction does to make designing mechanical things difficult -- it is still necessary.
     
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