It is commonly known that friction opposes motion. For example, if a block is sliding down a wooden surface with no forces other than friction acting on it, friction acts in the direction opposite to the block's velocity.
Let's say a car is driving at constant speed along a perfectly circular loop road. Centripetal acceleration works toward the center of the circle, but the car is moving forward along the loop, perpendicularly to the car's acceleration. As we have already established, friction should be in the opposite direction of the car's motion. But no- apparently it acts as the centripetal force that is perpendicular to the velocity.
What is going on with this discrepancy?
Edit: new comment
Edit: Furthermore, the book I am using says that, when the loop is banked, friction ceases to act centripetally and the centripetal force is provided entirely by the horizontal component of the normal force (neither friction nor, to my surprise, parallel force.). Why is that? Thanks!
Answer
Friction is in the opposite direction of motion.
When you spin your wheels, you are trying to push the road backwards. The reaction to that friction is to push your wheel forward.
When you turn, you are changing the angle the friction is acting. Instead of just trying to push the road backwards, it also tries to push it to the side opposite that you want to turn. The reaction from that creates a component of the force acting in the direction you want to turn, leading to the circular motion.
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