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Friction Formula

Discussion in 'After Hours Lounge (Off Topic)' started by Steve Ridges, Sep 15, 2003.

  1. Steve Ridges

    Steve Ridges Stunt Coordinator

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    Does anyone have a friction formula that accomodates surface area?

    The typical formula: F=N*mu, does not take into account the surface area.

    ie. Why is it easier to pull a rolled up rug, rather than an unrolled rug, they both have the same weight?
     
  2. Will Pomeroy

    Will Pomeroy Stunt Coordinator

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    Thats just because of the differences in surface textures. There is no formula that would accomadate all different types and variations of every kind of surface. The formula you gave does accomadate changes in surface area because when there is more area, there is less pressure, and vise versa; thefore the effect of friction is constant.
     
  3. Joe Szott

    Joe Szott Screenwriter

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    It isn't any easier to pull a rolled up rug compared to an unrolled one. It seems easier because of the way our hands/bodies are designed. If you could somehow grab the entire side of the rug at once and pull it would move as easily as if it was rolled up. But since your hands can only reach from around 0-4' easily (spreading arms out), grabbing a 2' rolled up carpet is easier than a 20' long side of carpet. In the 20' example, you just have a very awkward pinpoint grab on large surface, for the 2' rolled carpet it is much easier to contain and use your arms/back/legs/etc.
     
  4. Steve Ridges

    Steve Ridges Stunt Coordinator

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    So surface area plays no roll in the amount of friction?? That doesn't seem right. The reason I ask was that I was talking to a friend about tires on race cars. I had always thought that race cars used wide slicks because it increased the size of the contact patch wich would increase the friction and give more grip. If surface area doesn't matter, you should be able to put bicycle tires on an F1 car and still have it perform (assuming a strong wheel of course). Can you guys help me understand this?

    Thanks!
     
  5. Holadem

    Holadem Lead Actor

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    This kind of formula is used for systems modelled as point mass. For more accurate results, you will need to express a differential Friction as a function of area, then integrate. This is assuming the area has a reasonnable degree of regularity, so that it can be approximated with some sort of numerical method in the absence of a formula.

    --
    H
     
  6. Cary_H

    Cary_H Second Unit

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    If you are trying to arrive at the force required to overcome that of friction, F = Nu

    F is your friction force.( force required to overcome that of friction)
    N is your normal pressure between your two surfaces.(weight)
    u is your coefficient of friction between the two surfaces.
    (u = F/N)

    With this formula, area of contact is of no consequence.

    I'll have to give some more thought to the race car tire thing.
    One thing I would think is that the wider tire just might be the way you have to go to engineer the structural strength to withstand the loads it gets exposed to, and to dissipate enormous heat buildup.
     
  7. Joe Szott

    Joe Szott Screenwriter

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    Steve,

    As stated above, this formula is simply for the force (or work if you like) required to over come friction and move something. It has nothing to do with grip, traction, or anything else.

    The example you raised of a stock car on bicycle tires is still totally valid. The proper use of it in the formula above though would be if you were behind it trying to push it from a stanstill, would it roll any easier on stock tires rather than bike tires (assuming overall mass is same and bike tires could take the load)? Of course it would be every bit as hard to push, so the force you exerted would be the same regardless of tire surface area. So F=Nu still holds. To take this to an extreme, if you could actually get the car to balance on 4 tires made of razor blade thin materials, it would still requires the same effort to push it horizontally (car doesn't get any lighter), so F=Nu still.

    So why do racing cars have those big wide tires? From an online physics site I found:

    Because the size of the contact area is very important in car tires as the traction is dynamic rather than static; that is, it changes as the tire rolls along. The maximum coefficient of friction can occur anywhere in the contact area, so that the greater the area, the greater the likelihood of maximum traction. Thus, under identical load and on the same dry surface, the wider tire has a greater contact area and develops higher traction, resulting in greater stopping ability.

    Or another explanation of the same:

    It is true that wider tires commonly have better traction. The main reason why this is so does not relate to contact patch, however, but to composition. Soft compound tires are required to be wider in order for the side-wall to support the weight of the car. softer tires have a larger coefficient of friction, therefore better traction. A narrow, soft tire would not be strong enough, nor would it last very long. Wear in a tire is related to contact patch. Harder compound tires wear much longer, and can be narrower. They do, however have a lower coefficient of friction, therefore less traction. Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths. Wider tires, assuming all other factors are equal, commonly have stiffer side-walls and experience less roll. This gives better cornering performance.


    As an aside Steve, the greater surface contact s actally a detriment whent he road is wet because that changes the rolling/static friction coefficients. The same wide flat tire that helps you accelerate and corner on dry pavement is absolutely also the best hydroplanar in wet condidtions. That's why your 'normal' tires have grooves, so the rain/snow gets diverted into the grroves and allows the rest of the tire to stay in contact with the road. You always want your tires to roll as much as possible (rolling friction) as opposed to skidding (static friction.)

    Did this help at all?
     
  8. Steve Ridges

    Steve Ridges Stunt Coordinator

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    Thanks for the explination guys. Special thanks to Joe for taken the time to write all that out. That just the exact info I was looking for.
     

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