Question: I've been doing physics problems regarding cars for a while. I understand that there is a static friction (which appears when the wheel is rolling) and kinetic friction (which appears when the wheel is sliding). However, the way I'm visualizing it, static friction between the tire and road should not stop a car. In fact, when I asked this question to my teacher a long time ago, he said that it is actually the friction between the axle and the wheel that stops the car, and the road friction actually helps the car to move. But I know that when the car slips, the friction decreases and thus stopping time increases. How could this possible be linked to the axle? What is going on?

Answer: 
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> the way I'm visualizing it, static friction between the tire and road should not stop a car.
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Static friction is able to supply a force. If that force is opposite the direction of motion, it is able to stop the car. In the case of your car and the brakes, that's exactly what happens.

A torque from the brakes is applied to the wheel. This torque becomes a force against the ground. As long as the force is not too great, the wheel doesn't slip and the road supplies a force back on the wheel (which slows the vehicle).

> 
> I don't think the vehicle is slowed down by the static friction between the wheel and road. 
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And yet it is. To see that this is true, let's imagine a situation where we remove static friction. Drive the car onto a patch of ice where we assume the coefficient of friction drops to zero. The car continues to drive at the same speed.

When we stomp on the brakes now, a torque is still applied to the wheel, but now the force of static friction is zero. The car does not slow down and continues at the same speed. Only when (hopefully static) friction is present can we slow the car.

Kinetic friction is also capable of slowing the car, but we don't want that because we don't want to skid the tires.

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> ...that should be the static friction between the car and the road
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The only portion of the car that touches the road is the wheels/tires. That is the only part where friction can develop. So to say the friction between the car and the road or the friction between the wheels and the road is the same thing.
Your teacher's explanation is a bit thin. Firstly, there are several reasons that a car might slow down. Secondly, even when specifically focusing on internal friction, there are several more factors involved.

If there is kinetic friction
----------------------------

(if the wheels are sliding), then that friction is of course slowing down the car.

In the cases of static friction,
--------------------------------

it is correct that it is not the static friction that directly causes slowing. Only indirectly, as the static friction is a response to other factors. 

* **If you let go of the gas pedal and put the car in neutral**, ideally, the car will never stop. Realistically,
	+ there is friction, as you teacher says, in axles and axle joints, bearings etc.
	+ Also, realistically, the compression and expansion of soft rubber wheels requires work and "sucks" out energy, which is taken from the kinetic energy as well.
	+ Also, driving on, say, a soft road (think of a sandy beach) will similarly cause deformation of the surface and thus energy lost as work.

All this these non-ideal losses are usually combined into one umbrella term: **rolling friction** or **[rolling resistance](https://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html)**.

* **If you let go of the gas pedal but keep the car in gear**, the gearing system is still connected to the axle. Constantly driving the gearing is a tough task that causes a counter-torque, slowing down the car.

	+ This is known as [engine braking](https://en.wikipedia.org/wiki/Engine_braking), and is particularly useful in larger trucks.
	+ In typical electric cars, the axle system is connected to a one-way electrical generator system, so that the car's kinetic energy is converted [back into electrical energy](https://www.explainthatstuff.com/how-regenerative-brakes-work.html) by letting it drive the generator when the intention is to slow down. This is called [regenerative braking](https://en.wikipedia.org/wiki/Regenerative_brake).
* **If you push the brakes**, the counter-torque that slows down the car obviously comes directly from friction between the brake module and the wheel. [Depending on the type of brake](https://www.kwik-fit.com/brakes/information/how-brakes-work), this could be friction

	+ due to the brake pads in a brake drum being squeezed onto the wheel, or
	+ due to clamps pressing on a disc brake.