Friction Forces

<h1>Text Preview</h1> <p> Earlier in this lesson, we briefly talked about friction and its effect on moving objects. Friction is a force that exists between two objects that are in contact with each other, and always opposes motion. The amount of friction between two objects is characterized by the coefficient of static friction, mu, which is usually a number between 0 and 1 There are 2 varieties of friction forces. Static friction is a frictional force which prevents objects at rest from moving. The magnitude of the force of static friction is given by the coefficient of static friction, times the normal force, which, on a flat surface, is equal to the weight of the object. The subscript s on the mu denotes static friction. Once we exert enough force to overcome static friction, we have to deal with kinetic friction, or rolling friction. The formula for kinetic friction is similar to that for static friction. We denote the coefficient of kinetic friction with a subscript k. Usually, the coefficient of kinetic friction for an object is much smaller than the coefficient of static friction for that object. In other words, once an object starts moving, it requires less effort to keep it in motion. Let's revisit the inclined plane, this time taking into account the frictional force. Recall that in the absence of friction, the object slid down the incline because of gravity. Now, the force of gravity has to overcome the force of static friction to move the object down the slope. The forces acting on the stationary crate are gravity, pointing downward, the normal force, pointing in the positive y direction, and the force of static friction, pointing up the plane, in the positive x direction. Let's begin with the y-component of the forces. This equation is the same as the one we obtained in the absence of friction. We can solve this equation to obtain the magnitude of the normal force, which we will need to get the frictional force acting on the crate. The x-component of the net force has the force of gravity plus friction. Does the mass accelerate down the plane? That depends. If the component of the force of gravity can overcome the force of friction, then the object will move. The critical angle where this occurs can be found by setting Fx equal to zero and solving for theta.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>