Friction Kinetic and Static

Forces of Friction - arises from the electromagnetic forces between atoms and molecules at the surfaces of objects - is a force between two surfaces that are sliding, or trying to slide across one another - always works in the direction opposite from the direction the object is moving, or trying to move and always slows a moving object down

The Factors Affecting Amount of Friction 1. the normal force (N) 2. the materials from which the two surfaces are made / relative roughness of the surfaces in contact (coefficient of friction,  ) Note: The frictional force is independent of the contact area between the objects.

The frictional force. 1. The force of friction F f =  F n Normal force (N) Coefficient of friction Friction force (N)

Types of Friction Static friction Sliding friction Rolling friction

Types of Friction Air friction Viscous friction

2. The force of kinetic friction (moving object) The direction of the kinetic frictional force is opposite the direction of motion of the object it acts on.  The kinetic friction force is less than the maximum  static friction force. The direction of the static frictional force is along the contact surface and opposite in direction of any applied force. There are Two Main types of frictional force. 1. The force of static friction (no motion)

The plot below of the frictional force vs. the applied force illustrates some of the features of the frictional force. Note that the frictional force equals the applied force (in magnitude) until it reaches the maximum possible value.

time

Approximate friction coefficients ss kk Rubber on concrete Wood on wood Waxed wood on wet snow Joints in humans

The table below summarizes the main characteristics of the frictional force. Static FrictionKinetic Friction Symbolfsfs fkfk Direction opposite direction of applied force or impending motion opposite direction of object's motion Magnitude  sN sN kNkN

1.A 25.0-kg chair initially at rest on a horizontal floor requires a 365-N horizontal force to set in motion. Once the chair is in motion, a 327-N horizontal force keeps it moving at a constant velocity. Find the coefficient of (a) static and (b) kinetic friction between the chair and the floor. F s =  s F n (F n = mg)  s = F s /F n  s = 365/245  s = 1.50 Exercises on Friction  k = F k /F n  k = 327/245  k = 1.33

13 Balanced Forces The book is said to be at equilibrium.

14 Unbalanced Forces In this case, an unbalanced force acts upon the book to change its state of motion.

Unbalanced Friction Forces You push a 25.0 kg wooden box across a wooden floor with a force of 98 N. What is the resulting acceleration of the box? (0.20 is the coefficient of kinetic friction) M = 25.0 Kgu k = 0.20 F p = 98 N a = ? Draw a freebody diagram.

Unbalanced Friction Forces F net = Fa – F f ma = Fa – F f a = Fa – F f m Fg FaFa FNFN FfFf

Unbalanced Friction Forces What does the frictional force equal? So, a = F p – u k mg m = 98 N – (0.20)(25.0 kg)(9.8 m/s2) 25.0 kg = 2.0 m/s2 F f =  k F N