Force Friction

Friction The force that acts to oppose the motion between two materials moving past each other.

There are many different types of friction between surfaces. They include:

Static friction The resistance force that must be overcome to start an object in motion

Kinetic or Sliding friction The resistance force between two surfaces already in motion

Fluid friction The resistance force of a gas or a liquid as an object passes through. One example of fluid friction is air resistance

The resistance force between a surface and a rolling object Rolling friction The resistance force between a surface and a rolling object

Kinetic or Sliding friction In this unit we will deal only with kinetic and sliding friction The force of sliding friction between two surfaces depend on the normal force pressing the surfaces together, and on the type of surfaces that are in contact with each other.

Kinetic or Sliding friction The magnitude of this force is written as Force of sliding friction = (coefficient of friction) (normal force) Or Ff = μFN

Kinetic or Sliding friction Note: If an object is sitting on a horizontal surface, the normal force is equal to the weight of the object. The symbol μ (pronounced “mu”) is called the coefficient of sliding friction ( in other words, a large number for μ) means that the object is not likely to slide easily, while a low coefficient of friction (or small μ) is found between very slippery surfaces.

Units Because the coefficient of sliding friction is simply a ratio of the force of sliding friction to the normal force, it has no units.

When you are training its called resistance When you are racing its called drag

Example # 1 Robert is walking through the school cafeteria but does not realize that Ian (in front of him) has just spilled his carton of chocolate milk. As Robert, who weighs 420N, steps in the milk, the coefficient of sliding friction between Robert and the floor is suddenly reduced to .04. What is the force of sliding friction between Robert and the slippery floor?

Example # 1 Given: FN=420N μ =.04 Equation: Ff = μFN Robert is walking through the school cafeteria but does not realize that Ian (in front of him) has just spilled his carton of chocolate milk. As Robert, who weighs 420N, steps in the milk, the coefficient of sliding friction between Robert and the floor is suddenly reduced to .04. What is the force of sliding friction between Robert and the slippery floor? Given: FN=420N μ =.04 Equation: Ff = μFN

Example #2 While redecorating her apartment, Jessi slowly pushes an 82-kg china cabinet across the wooden dining room floor, which resists the motion with a force of friction of 320N. What is the coefficient of sliding friction between the china cabinet and the floor?

Example # 2 Given: m= 82kg g =10 m/s2 w= 820N = FN Ff= 320N Unknw:μ While redecorating her apartment, Jessi slowly pushes an 82-kg china cabinet across the wooden dining room floor, which resists the motion with a force of friction of 320N. What is the coefficient of sliding friction between the china cabinet and the floor? Given: m= 82kg g =10 m/s2 w= 820N = FN Ff= 320N Unknw:μ μ= Ff /FN

Example #3 At sea world, a 900kg polar bear named Freeberry slides down a wet slide inclined at an angle of 25º to the horizontal. The coefficient of friction between the bear and the slide is .05. What frictional force impedes the bear’s motion down the slide?

Example # 3 At sea world, a 900kg polar bear named Freeberry slides down a wet slide inclined at an angle of 25º to the horizontal. The coefficient of friction between the bear and the slide is .05. What frictional force impedes the bear’s motion down the slide?