We know that friction helps in driving a car, but does this mean that a car can move faster on rough surfaces? Since the coefficient of friction is higher on rough surfaces?
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3$\begingroup$ Note that friction does more than help drive a car. It is the force that accelerates the car. $\endgroup$– garypCommented Nov 26, 2015 at 3:03
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$\begingroup$ No...friction slows down the automobile...the most pronounced being the friction in the form of heat created by the engine and drive train. What propels the inertial mass of an automobile is first and foremost its inertial mass (the bigger the better) and then what is to be consisted (the fuel.) The control surfaces (steering wheel, hydraulic actuators, linkages, tires, etc) are critical in making sure Das Auto drives in a straight line...and with minimal friction actually (four tires of the same size arranged in perfect parrarllel with one another.) $\endgroup$– Doctor ZhivagoCommented Nov 5, 2016 at 20:20
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$\begingroup$ @user14394 Cars do not drive with minimal friction. Rather the contrary. You want high friction when accelerating fast. Otherwise explain to me why ice is not the best surface to accelerate on. In non-ideal situations, friction in gearing and axles etc. do slow down the car as you explain, but these are not what are being asked about since these are not the frictions that propel the car forward. The static friction on to the ground is propelling it forward (because the engine forces the wheels around), and this friction does not cause energy loss (it does no work). $\endgroup$– SteevenCommented Jun 28, 2017 at 22:08
5 Answers
The coefficient of friction $\mu_s$ might be higher for tires on rough surfaces, yes, but as you said yourself, it is friction $f_s$ that thrusts the car forward.
(And we are talking about static friction throughout, since we are talking about rolling wheels.)
And the coefficient of friction is not equal to friction. High coefficient of friction does not mean high friction.
It only means that there can be high friction. If necessary. It means that if the car starts gripping harder in the asphalt, then the asphalt can hold on. But only if. If the car grips the same amount (if you drive in the same manner) then you get the same friction, no matter what the coefficient of friction is (as long as it is not too small).
Mathematically that is shown in the formula for static friction:
$$f_s\lt \mu_s n$$
The friction $f_s$ does not have to rise, when $\mu_s$ is high. It can if it has too, but it doesn't if it doesn't have to. The coefficient of friction (times the normal force) just determines the maximum. That's all.
The friction should be optimum. The cars surely run better on roads than in sand. The cars have tires so the friction is less and the friction of cars mainly depend on normal force than roads. More rough roads will increase grip during acceleration but as the car starts moving in a constant velocity the roads will grip the tire and drag. Cars will not be fast on rough roads.
The same problem is with F1. Tire gives grip during acceleration but grips road. To improve this Aerodynamics is the answer.
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$\begingroup$ "The cars have tires so the friction is less" What do you mean by this sentence? That cars have tires in non-ideal situations usually only makes friction higher and energy losses larger. As well as less wear and tear and more comfort. Good point about the wish for less roughness and thus less friction when driving fast at constant speed - that is reaching into the topic for rolling resistance, which is usually neglected. $\endgroup$– SteevenCommented Jun 28, 2017 at 22:24
There is a maximum fricction force, and is F=uN. u is the coefficient of friction and N the paralel component of the weight to the normal. If the car is at rest, a higher coefficient makes a better accelration. But if the car has a velocity v, only will be a diference between coefficient at a range. If U is the maximu coefficient at that range, coefficients highers than U will not make any diference in the velocity of the car.
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$\begingroup$ "If the car is at rest, a higher coefficient makes a better accelration" What do you mean by this? If the coefficient is 10 or if it is 100 makes no difference, if e.g only a value of 5 is needed. A higher coefficient makes a higher acceleration possible, but if you accelerate the same then a higher coefficient makes no difference (as long as it is not too small). You don't get a "better" acceleration, whatever that means. $\endgroup$– SteevenCommented Jun 28, 2017 at 22:19
There are many types of friction in an automobile...this question dealing with rolling resistance. Most of the heat generated comes from the inertial mass of the vehicle itself and not from the road. Since a paved road is smooth and tires circular there is much less friction on a smooth surface than a rough one so the answer which is also true empirically is no...a rough surface provides far more friction as the tires cannot dissipate the heat nearly as efficiently given weight and the suspension system.
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$\begingroup$ "Most of the heat generated comes from the inertial mass"... From the inertial mass? And not from gearing friction, axle friction, tire deformation, fuel consumption? $\endgroup$– SteevenCommented Jun 28, 2017 at 22:15
I will base my answer on simple rolling motion- as car starts velocity of center of mass of Tyre reaches v=rw state. if i press breaks friction acts backward to stop the car. if i press accelerator friction acts forward to reach v=rw state. this explanation is high school level more deep discussion requires more insight.