救命啊! 我諗到頭爆都諗唔到向心力點解係磨擦力!!!

1. 點解會有個磨擦力向中心? (個velocity向前, friction唔係應該向後ma?)
When a car turns a corner, there exist two frictional forces in two directions. One the the "usual" frictional force that is opposite in direction to the speed of the car at that instant. The other one is perpendicular to speed of the car, which helps the car to turn a corner. This is the "lateral frictional force". It would not appear if the car travels in a straight line.

Just imagine when a car turns a corner, it tends to slide outward from the circular path because (according to Newton's First Law) it tries to keep its motion in a straight line. If the road surface is rough, the slidding outward motion would be counter-balanced by an inward frictional force (as friction is always opposes motion). This is how lateral frictional force arises. A car would not be able to turn a corner if the road surface is perfectly smooth, such that lateral friction doesn't arise.

2.點解向心力=磨擦力???? 平常d motion唔係同磨擦力相反方向架咩??
"Centripetal force" is only a name given to whatever force that helps an object to move in a circular path. In this example, the "lateral frictional force" helps the car to turn. Hence, the "lateral frcitional force" itself is the "centripetal force".

3.向心力和離心力係咪一對力黎架? 離心力係咪acting on air?
I don't suggest you bothering about "centrifugal force"(離心力) at the moment, as it may confuse you. Centrifugal force is NOT a real force, and is only experienced in a "non-inertia reference frame". The passesnger inside a car turning a corner would experience suh centrifugal force.

Clearly, centrifugal force is NOT acting on air.

From what you wrote, I suppose you were all the time thinking about linear motion - car moving forward, the friction is backward. Of course, this is correct.

Now, you are learning / considering a circular motion
which means that the car is NOT moving to ONE single direction with a certain velocity, it is turning instead.

When the car is moving in a circular path, the only SIDEWAY force is the friction which cause the car to move in a circle. There is no other forces. Therefore, the friction must be the centripedal force.

From your remark of "我基本上成個圓周運動都唔明, 諗左幾日都諗唔掂, 求好心人教下我 ", I am afraid it is not right for you to ask the first two questions. I suggest you to ask a classmate to help explain to you in person - face to face. I suppose it will be very difficult to explain to you on this forum.