Friction depends on force, not surface area
- Thread starter WinkOsmosis
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johnjohn320
Diamond Member
- Jan 9, 2001
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F = ma and no where in ma is surface area.
in general, friction does not depend on surface area, but exceptions exist when other factors are introduced.
More contact patch = more abilty to hold traction (for a vehicle anyway). Surface area does matter.
Think about it
Say you had held a plate glass window between two fingers of your hands. Only using one finger per hand to touch the glass. Think how much force it would take to keep that window from slipping through your fingers.
Now try it using the whole palm of both hands.
Ahhh, much less force needed to hold the same window.
We know the window didn't suddenly get lighter so obviously the area of the contact point DOES matter and is a factor.
More contact area = more resistance to being moved (slipping) at a given coeffecient of friction.
Think about it
Say you had held a plate glass window between two fingers of your hands. Only using one finger per hand to touch the glass. Think how much force it would take to keep that window from slipping through your fingers.
Now try it using the whole palm of both hands.
Ahhh, much less force needed to hold the same window.
We know the window didn't suddenly get lighter so obviously the area of the contact point DOES matter and is a factor.
More contact area = more resistance to being moved (slipping) at a given coeffecient of friction.
johnjohn320
Diamond Member
- Jan 9, 2001
- 7,572
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F does not stand for friction. You wrote the formula for "total force."
Also, mass often depends on surface area. Something bigger is generally heavier.
I really don't know much about physics, but don't most successful drag cars run on really wide tires?
right, other factors
any intro to physics text will tell you that SA is not related to the frictional coefficient. but that's using perfect models and crap. with cars, there are a slew of previously-not-considered factors that will make wider tires grab more.
Possibly.. but that certainly does not answer the original question, and is a broad generalization. (And I would not go around saying "mass depends on surface area", there may be a relationship between the two (tenous at best, with dependecies on other variables), but there isn't a "dependency").
It's been a while since I took physics, but friction depends on force AND a coefficient. The coefficient of friction for a tire is dependent on its contact patch. Or I suppose you could find the coeffienct of rubber to asphault, and integrate over the surface of the contact patch etc.. though I have no idea how the tread pattern affects that.
silverpig
Lifer
- Jul 29, 2001
- 27,703
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Wrong. Pressure depends on surface area. Force does not. You have to exert the exact same amount of force on the glass whether you use your palms or your fingers. The PRESSURE on your fingers is greater because there is a certain force on a very small surface area.
silverpig
Lifer
- Jul 29, 2001
- 27,703
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And here's my explanation:
F(friction) = mu*F(normal between surfaces)
Despite what some may think, a larger surface area DOES NOT increase the force between the tires and the road. Whether you put skinny bicycle tires or huge 24" wide mothers on won't make a single bit of difference as to how much force there is between the tires and the road. There are only two factors that will affect that:
1. The actual weight of the car, and
2. Aerodynamic downforce/lift
(assuming you're just travelling on flat land that is).
We are therefore left with only one term that could possibly affect the friction. mu.
mu is (as already stated in previous posts) called the coefficient of friction. It is the result of a large number of varying factors (materials, roughness, geometry, etc etc).
Also, a simple mu*F isn't what you are looking for. That would be for moving friction and, unless you are considering spinning the tires or locking up the brakes, this isn't what you want to deal with. If you're talking about pure grip, you want to deal with the static friction which in most cases is slightly greater than the moving friction. It also depends on a great number of factors, surface area being one of them I suppose.
To be fully honest though, I couldn't tell you exactly why fat tires grip better other than the pseudo-reasons I've given above, but I can tell you that they do not in any way increase the force between the car and the road.
F(friction) = mu*F(normal between surfaces)
Despite what some may think, a larger surface area DOES NOT increase the force between the tires and the road. Whether you put skinny bicycle tires or huge 24" wide mothers on won't make a single bit of difference as to how much force there is between the tires and the road. There are only two factors that will affect that:
1. The actual weight of the car, and
2. Aerodynamic downforce/lift
(assuming you're just travelling on flat land that is).
We are therefore left with only one term that could possibly affect the friction. mu.
mu is (as already stated in previous posts) called the coefficient of friction. It is the result of a large number of varying factors (materials, roughness, geometry, etc etc).
Also, a simple mu*F isn't what you are looking for. That would be for moving friction and, unless you are considering spinning the tires or locking up the brakes, this isn't what you want to deal with. If you're talking about pure grip, you want to deal with the static friction which in most cases is slightly greater than the moving friction. It also depends on a great number of factors, surface area being one of them I suppose.
To be fully honest though, I couldn't tell you exactly why fat tires grip better other than the pseudo-reasons I've given above, but I can tell you that they do not in any way increase the force between the car and the road.
So no one knows why fatter tires grip better. Could it be that bumps in the surface of the rubber grip better with the bumpy surface of the road when there are more of both in contact?
Two reasons:
1) For offroad vehicles, a greater surface area equates to lower pounds per square inch of force, making the vehicle less likely to sink in sand or mud. Serious offroaders "air-down" their tires to further increase ground-contact area.
2) This one from Russ Steele, my high school physics teacher way back in 1982. As previous poster pointed out, if the force (weight of the vehicle) remains constant, the friction will remain constant regardless of surface area. Mr. Steel told us drag racers use fat tires to reduce wear. As rubber is abraded off the tire, the diameter decreases. Fat tires have a lot more rubber to wear off than skinny tires.
1) For offroad vehicles, a greater surface area equates to lower pounds per square inch of force, making the vehicle less likely to sink in sand or mud. Serious offroaders "air-down" their tires to further increase ground-contact area.
2) This one from Russ Steele, my high school physics teacher way back in 1982. As previous poster pointed out, if the force (weight of the vehicle) remains constant, the friction will remain constant regardless of surface area. Mr. Steel told us drag racers use fat tires to reduce wear. As rubber is abraded off the tire, the diameter decreases. Fat tires have a lot more rubber to wear off than skinny tires.
Garlic breath has hit the nail on the head there...
The surface area is actually irrelevant to the friction. If you put a car on a single pin, the maximum* frictional force at the head of the pin is equal to the sum of the maximum frictional forces at the tyres.
Tyres are made large for the following reasons:
Supporting weight. Jumbo jets have massive tyres to spread the massive weight out. If you put too much force on a tyre, it will burst. Also, pressure becomes an issue. Off-road driving needs a decent surface area to be in contact to stop a car sinking into the mud.
* Friction is actually less-than-or-equal-to the product of the Normal Reaction and the Coefficient of Friction (mmm, pedantry)
The surface area is actually irrelevant to the friction. If you put a car on a single pin, the maximum* frictional force at the head of the pin is equal to the sum of the maximum frictional forces at the tyres.
Tyres are made large for the following reasons:
Supporting weight. Jumbo jets have massive tyres to spread the massive weight out. If you put too much force on a tyre, it will burst. Also, pressure becomes an issue. Off-road driving needs a decent surface area to be in contact to stop a car sinking into the mud.
* Friction is actually less-than-or-equal-to the product of the Normal Reaction and the Coefficient of Friction (mmm, pedantry)
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