# Unbanked Curve Motion: Friction vs Intuition

• JJ__
In summary: Beyond that limit, static friction can only provide a force that is opposed to the direction of motion.
JJ__
This is just a conceptual question. I get that when a car is turning on an unbanked curve, the friction provides the centripetal force. I don't understand why this is though. I thought friction is supposed to oppose the direction of motion. But that would imply that the direction of motion points directly out from the circle. But intuitively it seems like the direction of motion would be tangent to the circle (i.e. perpendicular to the friction)...

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The same question arises if I walk (or cycle, or drive) across a slope in a direction perpendicular to the slope (or at any angle not parallel to the slope.)
I think the way to look at this is to see that, in order to move in such a way, I need to apply an appropriate force to the surface and friction acts opposite to this force. That force will involve components up the slope and in the direction of motion.
The deceptive element is that simple friction questions involve objects moving parallel to any slope, so that the force we apply is parallel to the motion.
Friction does not always act in the direction of motion. Rather perhaps, the net force, including a contribution from friction, acts in the direction of any acceleration (or negative acceleration, aka deceleration.)

JJ__ said:
I thought friction is supposed to oppose the direction of motion.
Kinetic friction opposes the direction of relative motion. But the surface of the tire and the surface of the road are not in relative motion. The wheels are rolling so that the contact patch does not slip on the pavement. Instead, we are dealing with static friction.

Static friction provides whatever force is needed to prevent relative motion between two surfaces. The contact patch is not slipping. There is no relative motion. The wheels are free to roll forward or backward. Accordingly, little or no static friction is needed parallel to the car's motion to prevent slippage fore and aft. However, unless the tires slip right or left, the car is constrained to move along the curved path where the wheels point. Static friction acts to prevent the tires from slipping right or left away from this path.

Of course, static friction can only provide force up to the limit imposed by the coefficient of static friction.

## 1. What is unbanked curve motion?

Unbanked curve motion refers to the movement of an object, such as a car or bike, around a curve without any banking or incline on the surface. This means that the surface is flat and does not provide any additional support or assistance to the object as it moves around the curve.

## 2. What is the role of friction in unbanked curve motion?

Friction plays a critical role in unbanked curve motion as it is the force that acts between the object and the surface, providing the necessary traction for the object to maintain its motion around the curve. Without friction, the object would slide or skid instead of following the curve.

## 3. How does intuition play a role in unbanked curve motion?

Intuition refers to our natural understanding or perception of a situation, and in the case of unbanked curve motion, it can play a role in how we anticipate and adjust our movements to navigate the curve. Our intuition helps us to make quick decisions and adjustments to maintain our balance and control while moving around the curve.

## 4. What are some factors that can affect unbanked curve motion?

There are several factors that can affect unbanked curve motion, including the speed of the object, the shape and size of the object, the surface material and condition, and the amount of friction present. These factors can impact the object's ability to maintain its motion and can also affect the amount of force and energy required to move around the curve.

## 5. How can we improve our understanding of unbanked curve motion?

Improving our understanding of unbanked curve motion requires a combination of theoretical knowledge and practical experience. Studying the principles of physics, such as Newton's laws of motion and the concept of friction, can provide a foundation for understanding the mechanics of unbanked curve motion. Additionally, practicing and observing the motion in real-life situations can help us develop a better intuition and improve our ability to navigate unbanked curves.

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