# A conceptual doubt about friction acting on a car

Suppose a car is at rest and you apply the accelerator, then the wheels are applying a horizontal force on the road so according to Newton's third law an opposite force (friction) pushes you forward. From what I understand no matter how much force you apply (hit the accelerator) the maximum force that can push you forward is the limiting friction. But then since the car is going with an acceleration forward friction should again act backward and there will be no net force, so the car doesn't move. How is this possible?

But then since the car is going with an acceleration forward friction should again act backward

Friction does not suddenly change direction, it continues to act in the forward direction.

Friction does not act to oppose motion, it acts to oppose slipping. The tires are tending to slip backwards, so friction points forwards.

The wheels of the car applies a force on the road. The horizontal component of the force applied by the road on the car is the friction.

Now let's see what actually friction is...

Friction is the force which opposes relative slipping between the two surfaces in contact.

So now let's assume there is no ground beneath the car...the wheels will tend to slip backward. So to oppose the relative motion the ground applies friction in forward direction and pushes the car forward.

Hope this gives you a clear concept of friction.

There are two possible friction forces acting forward on the wheels a consequence of Newton's 3rd law in response to the drive torque: static and kinetic.

Static friction prevents relative motion (sliding or slipping) between the wheel and the ground and acts forward to enable the car to accelerate without slipping/skidding. That static friction force matches the force the wheel applies backward on the road due to the torque on the wheel up until the maximum possible static friction force is reached. That is what you call the "limiting friction". The limiting static friction force on a given wheel is

$$f_{static-max}=\mu_{s}N$$

Where $$\mu_{n}$$ is the coefficient of static friction between the wheel and the road, and $$N$$ is the amount of the weight of the car supported by the wheel.

If the limiting static force is exceeded, the wheel slips/skids and the friction force becomes kinetic. The kinetic friction is

$$f_{k}=\mu_{k}N$$

where $$\mu_{k}$$ is the coefficient of kinetic friction between the wheel and the road. The kinetic friction force still acts forward on the wheel enabling acceleration, albeit with slipping/skidding.

But since $$\mu_k$$ is generally less than $$\mu_s$$, the kinetic friction force is generally less than the limiting static friction force. That means the acceleration will be less with kinetic friction than static friction, for the same applied drive torque to the wheel. And that's because some of the rotational kinetic energy developed by the drive torque is dissipated as heat in slipping/skidding when the friction is kinetic.

Hope this helps.

• This is not even an answer to the OP's question. Dale's answer is much more applicable—the issue is the direction of the friction, not the type or magnitude, as you bring in. Commented Sep 22, 2023 at 2:57
• @naturallyInconsistent In the very first sentence of my answer I stated that both static and kinetic friction act forward. My answer does not conflict with Dale's. It only goes further to try to help the OP understand the influence of kinetic versus static frictions on the acceleration of the car. Commented Sep 22, 2023 at 3:21

There are two horizontal forces acting between the tyre and the ground at the point of contact.

The force of the ground on the tyre (car) which accelerates the car (the system) in a forward direction.

The force of the tyre on the ground (N3L).

Note that only the first force acts on the car and that is the force which accelerates the car.

The consequence of the second force you may have noticed when a car accelerates away on a gravel drive and the stones from the drive fly off in the opposite direction to the motion of the car.