Kind of confused about friction ...?

Question

It just gives me a headache to think about it sometimes. For example, a car is moving forwards and suddenly brakes. My textbook says that there's a backward friction force of the road on the tires, but a forward friction force of the tires on the road.

Can someone please explain what this means in simple terms? Two friction forces in opposite directions kind of confuse me. I also thought that the friction force has to be in the opposite direction from the applied force. It's really bugging me because I know that this is probably simple but I'm just not getting it. (It's just grade 11 physics so please don't get into too complex stuff, we still always assume that there's no air resistance and such)

Answer 1

the tires are aplling friction on the road to move and the road is applling a force trying to stop the tires due to gravity. It's like wrestling ones trying to beat the other up the road is a two year old and the car is a Sumo Wrestler. But the car wins because all its force is in one stop the whole car but gravity's force is spread from the center of the Earth out

Answer 2

Friction occurs whenever two objects are in contact, whether the objects are  stationary or in motion.  Friction will always act on each surface, caused by the other surface.  As you drive, whether you are breaking or not, there is always going to be friction from the road acting on the wheels, attempting to slow the car down.  This is why unless we are trying to brake, we have to keep our foot on the gas, even if just slightly.    

Now, according to Newton's third law, all forces have an equal and opposite reaction.  No matter how big or small the action force is (in this case the force of friction acting on the car), there will be an equal force opposing the force of friction.  So in this case, there is a reaction force acting from the tires onto the road.  With this, all forces are balanced out.

Answer 3

Well I'm going into grade 12 AP phsyics... but after having a mediocre 11th grade physics teacher.

Basically

No surface is perfectly even. There's microscopic bumps and ridges that collide during motion. The tire is uneven. The road is uneven. The rubber of the tires is moving on the road. The 2 both have microscopic ridges, bumps, and imperfections on them that are hitting each other. Thus, a force is required to overcome these obstacles.

Since basically the road is moving backward the the car is moving forward, there are 2 directions of friction. You really don't have to be able to understand much of that. Your textbook seems to be over complicating it (we never used the book).

All you should have to know is that there's static and kinetic friction.

Static friction must be overcome to start motion. Kinetic friction must be overcome to continue motion after motion has started.

Answer 4

you need to separate the two things, because the two forces don't apply on the same object!

Imagine the road as a carpet on a slippery parquet: if you run and suddenly stop you act on the carpet with the friction of your socks, so the carpet might move forward. Obviously a road won't , because it is better anchored (luckily ;))

Back on topic: the force act on you to stop you, and on the road to move it forward: they're two backs of the same thing.

You want to go on but the road act on you with the friction and stop you: the same friction try to move the road on.

Parallel on magnets (geomag, for example): if you put two magnets not too far away the force would act on them BOTH and make them go together.

Parallel 2: gunshot. There is a chemical reaction, there, the explosion, that wants the bullet and the gun to go in different directions: so the explosion act on the bullet and make it start, AND on your elbow to make you go backwards.

Hope it is clear, i can try better...

Cheers, D