# What force stops a car?

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?

the way I'm visualizing it, static friction between the tire and road should not stop a car.

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.

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.

...that should be the static friction between the car and the road

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.

• I don't think the vehicle is slowed down by the static friction between the wheel and road. Unless I'm misunderstanding what you are saying. – Dude156 Jun 28 '19 at 20:02
• Shoot I'm sorry, I haven't slept much haha, I meant the static friction between wheel and road shouldn't slow down the car. Rather that should be the static friction between the car and the road (a sum of all braking and everything else). – Dude156 Jun 28 '19 at 20:05
• @Dude156, added a bit. – BowlOfRed Jun 28 '19 at 20:09
• I think you can refer to Steeven's answer and ensuing discussion to understand where I'm coming from. I think if we were in a room we would say the same thing. Also, by static friction between car and road I mean the braking, the gear, and everything else. – Dude156 Jun 28 '19 at 20:14
• Actually, maybe I'm more lost than I first thought. Are you saying that the braking force would be equal to the static friction force? – Dude156 Jun 28 '19 at 20:19

Imagine a car just going along at constant speed. Do your free-body diagram. The net force on the car has to be zero. So the wheels have to be exerting on the ground (net at least) only a vertical force.

Now imagine the car decreasing speed. There has to be a force opposite to the velocity. Free-body time again. The force on the ground has to include a force component that opposes the motion of the car. The wheels are the part touching the ground, so they must be supplying that force.

Remember your Newton's laws. To stop the car must be acted on by an external force. If you call the wheels "part of" the car, then the stopping force has to be applied by the ground. Meaning the wheels have to push back exactly as hard.

If you call the wheels "not part of" the car, then you can describe it as the braking mechanism applying a force to the wheels. Then it's "the wheel's problem" what it does with that force. But in that case, the car is stopped by friction between the brake mechanism and the wheel. It may be that your teacher is trying to get you to think that way.

• Oh I see now! So basically we are just considering the force of the brake mechanism on the car as a whole instead of at the wheel level right? But then why would the weight of the car change anything (normal force and all that)? Surely the brake should not be affected by the weight of the car right? Also, shouldn't we describe the brake system as kinetic friction then? – Dude156 Jun 28 '19 at 18:43
• @Dude156 The force of the brake pads on your rotors is dependent on the hydraulic pressure on them. This is the normal force used to calculate the force of friction (not the force of the car being pulled down by gravity). It is definitely kinetic friction (until the car comes to a stop). This force is then transmitted to the wheels (typically thought of as a torque, since it's a rotating thing), and the wheels then apply force to the ground. That latter force is static friction, since the wheels are not slipping. – Cort Ammon Jun 28 '19 at 22:41

Have you ever tried to lift yourself up by pulling your own hair? Go ahead try it. Try it really hard. You should be able to levitate a couple of inches, right? What? You got a handful of hairs in your hand and a bald patch in your skull? It should serve you right. Hopefully, you will learn that internal forces cannot change the total momentum.

The friction of the gears, engine, etc. are internal to the car. They cannot change the momentum of the car. For all we care, replace the gears, engine, and breaks with a couple elves and other mythical creatures (a.k.a. hidden variables). As long as those hidden variables are internal to the system (car) they cannot change the momentum of the car.

If you insist that friction cannot stop the car, I dare you to drive really fast on an icy road where there is almost no friction. Hooray for Darwinism.

• Wow thank you for enlightening me about conservation of momentum. You must be some sort of genius. – Dude156 Jun 28 '19 at 20:39
• Please edit your answer and change nothing (so I can revote), this helped a bunch. I'm sorry for the aggressive comment. I see exactly what you mean now. – Dude156 Jun 28 '19 at 21:35

cars stop with their brakes, which produce friction at the inside of the wheel assemblies as the wheels rotate. This friction force retards the rotation of the wheels and dissipates the kinetic energy of the car into heat in the brake parts.

since the wheels are in rolling contact with the pavement, and since the wheels are being slowed by the brakes, the pavement is pushing back against the wheels at their contact point with the pavement in a direction that opposes the movement of the car. So the car as a whole slows down.

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.

• 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, 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 by letting it drive the generator when the intention is to slow down. This is called regenerative braking.
• 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, 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.
• @Dude156 No, be aware not to confuse static and kinetic friction sources. When we say static friction, we mean non-sliding surfaces. While we see static friction between wheel and road, we also at the same time see kinetic friction between brake pads and wheel, when we push the brake. And possibly also some of the other factors mentioned at the same time (such as rolling resistance). Those are different types of friction and different sources of friction and have not necessarily got anything to do with each other. – Steeven Jun 28 '19 at 18:52
• @Dude156 Aha, I see your point now. There is indeed a connection between the static friction between wheel and road, and the internal resistances. For instance, when you push the gas pedal, the engine applies a torque that forces the wheel up in angular speed. The wheel might slide on the road, so the static friction must change to still avoid sliding. Similarly, when you brake (and when other slowing-down effects are active), a counter-torque is applied to the wheel forcing them to slow down angular speed. Friction must again adjust to keep it from sliding. – Steeven Jun 28 '19 at 19:32
• @Dude156 The limit for static friction does not limit the speed. It only limits the acceleration. You could imagine a very, very fast car with no push on neither the gas pedal nor the brakes. If the wheels are already up in angular speed to match the ground speed, then there is no static friction at all. It is zero, because there is no tendency for sliding between wheel and road. Such tendency only comes, when then wheel angular speed is changed. And only then will static friction matter. What will limit the speed, will thus instead be other things, like material durability. – Steeven Jun 28 '19 at 19:53
• Thanks for pointing that out. That's what I was thinking, but the clarification enlightened the fact that you don't need static friction if moving at constant angular speed (though how you would get there without an initial push is impossible haha). Thanks a bunch for the help! – Dude156 Jun 28 '19 at 20:01
• @Dude156 Yea, well I guess you can just accelerate really slowly for a very long time :) You are welcome. – Steeven Jun 28 '19 at 20:08