Physics: Uniform Circular Motion Question?

Question

If a car is moving around a circular track in uniform circular motion, with friction present, and a free-body diagram is drawn showing all the forces on the car (3 total forces), then why does the force of the friction, which is parallel to the surface always have a perpendicular or normal force, which is of course perpendicular to the force of friction? I know that there is a force of gravity downward which acts as the opposing force of the earth pushing up on the car, but HOW IS THE FORCE FROM THE TRACK on the car RELATED TO THE FORCE OF FRICTION, AND WHY ARE THE TWO DIFFERENT FORCES PERPENDICULAR. Thanks.

Answer 1

This is a good question.

First, "why are the two forces perpendicular"? The short answer is, that's the nature of friction (at least, "contact friction," as opposed to, say, air friction). The Normal Force is always (by definition) perpendicular to the contact surface, and the friction force is always _parallel_ to the contact surface.

On a microscopic scale, the two surfaces will look very "bumpy," with many "mountains" and "valleys". The normal force causes the "mountains" from one surface to fit into the "valleys" of the other surface, and the two surfaces literally become welded together in various spots where the contact is sufficiently close. The strength of the microscopic welds is enough to resist sideways motion (up to a point), and that's the source of the friction.

In the case of the car, the circular motion is in fact caused by friction, but it also obviously has something to do with the fact that the front wheels are turned at an angle. (After all, if the wheels were aimed "straight", then you suddenly wouldn't have this mysterious force pulling the car sideways.)

If you could look at the front tires closely during a turn, you would see that, at the place where the rubber contacts the road, the rubber in the front tires is distorted into a slight "S" shape. As the elastic rubber tries to snap back into its normal shape, it exerts a sideways force on the road, which responds by exerting an equal and opposite force on the tire. The nature of that force is friction, and its direction happens to be toward the center of the circle.

Answer 2

n physics, uniform circular motion describes the motion of a body traversing a circular path at constant speed. The distance of the body from the axis of rotation remains constant at all times. Though the body's speed is constant, its velocity is not constant: velocity, a vector quantity, depends on both the body's speed and its direction of travel. This changing velocity indicates the presence of an acceleration; this centripetal acceleration is of constant magnitude and directed at all times towards the axis of rotation. This acceleration is, in turn, produced by a centripetal force which is also constant in magnitude and directed towards the axis of rotation. In the case of rotation around a fixed axis of a rigid body that is not negligibly small compared to the radius of the path, each particle of the body describes a uniform circular motion with the same angular velocity, but with velocity and acceleration varying with the position with respect to the axis.