Friction Definition of Friction Friction is a force that opposes the direction of motion of an object. The Force Needed to Slide an Object To slide an object, the force of friction must be balanced. Friction is always just a % (fraction) of the objects weight. To push an object along (slide it), it is necessary just to overcome the force of friction, NOT to overcome its weight. The Force Needed to Slide an Object To slide an object, the force of friction must be balanced. Friction is always just a % (fraction) of the objects weight. To push an object along (slide it), it is necessary just to overcome the force of friction, NOT to overcome its weight. Friction depends on the type of surfaces rubbing, the force pressing the surfaces together as well as molecular attractions between the sliding objects. Measuring Friction Friction is measured by attaching a spring balance to an object and noting its reading while the object slides. Since a uniformly-sliding object has the friction force and applied force balanced in opposite directions, the magnitude of the applied force is the magnitude of the friction force. The Normal Force When an object is contacting a surface, the normal force is the force perpendicular to the contacting surface that is acting from the surface onto the object. The normal force is a reaction force to the objects perpendicular force on the surface. Examples of Normal Forces Coefficient of Friction The coefficient of friction is the ratio of the force of friction to the normal force. = F friction / F normal The coefficient of friction shows the fraction of an objects weight that friction is in a given situation. Static and Kinetic (Dynamic) Friction Static friction is the amount of force that must be overcome to start an object moving. Kinetic friction (dynamic friction) is the force that must be overcome to keep an object moving steadily. Comparisons of Coefficients of Friction What Type of Shoe Sole? If great traction (large static friction) force is needed (like in many sports), a sole with a large coefficient of friction is chosen. If a small degree of traction (lower static friction) is needed (like in dancing), a sole with a smaller coefficient of friction is chosen. Hookes Law Hookes Law states that the extension of an elastic body (like a spring) is directly proportional to the force applied to it. Hookes Law F = kx, where F if force in newtons, k = the springs/ elastics constant and x is the increase in length in metres The graph of force versus extension is a straight line which will pass through the origin. Hookes Law Hookes Law states that the extension of an elastic body (like a spring) is directly proportional to the force applied to it. F = kx, where F if force in newtons, k = the springs/ elastics constant and x is the increase in length in metres Adding and Subtracting Vectors Hook up the vectors tip-to-tail. The force sum or resultant force or net force is the vector drawn from the open tail to the open tip. Adding and Subtracting Vectors A A A A A A A A A A A