What is friction?

• Friction is a?force?between a surface and an object that acts to resist motion of the object across the surface
• On a horizontal surface with no forces acting on a stationary object other than?gravity?and the surface's?normal reaction force, there is no force of friction
• If an additional force acts on the object?parallel?to the surface then friction manifests as a force in the?opposite direction
  • Friction always acts in a direction?parallel?to the?surface
  • Friction acts in the?opposite direction?to the?resultant force?that would cause the object to move?parallel?to the surface
  • Up to a certain?limiting value?(see next section) the force of friction?prevents motion?entirely
  • Beyond that?limiting value?friction is a force of?constant magnitude?in the?opposite?direction to the motion of the object

   

What Is the coefficient of friction?

• The magnitude of the frictional force satisfies the inequality:

F?≤?μR

where?F?is the force of friction,?R?is the normal reaction force between the object and the surface, and?μ?is the?coefficient of friction?between the object and the surface

• • If there is no friction then?μ=?0?and the?surface?is?smooth
  • Otherwise?μ?>?0

   

• The maximum possible frictional force is given by the equation

FMAX?=?μR

where?FMAX?is the maximum possible force of friction, and?R?and?μ?are as above

• FMAX?serves as a?limiting value?for mechanics problems involving friction 
  1. If the object is stationary?and the resultant of other parallel forces to the surface is?less than or equal to?FMAX?then
     • the object?remains stationary
     • the frictional force is always?equal?and?opposite?to the resultant of the other horizontal forces
     • if the resultant of other horizontal forces is?equal?to?FMAX?then the object is said to be in?limiting equilibrium

      

  2. If the object is stationary?and the resultant of other horizontal forces becomes?greater than?FMAX?then
     • the object begins to?accelerate?in the direction of the resultant force
     • the frictional force is equal to?FMAX?with a direction always opposite to that of the object's motion

      

  3. If the object is in motion parallel to the surface?then (regardless of any other forces) the frictional force will always be?equal?to?FMAX?with a direction opposite to that of the object's motion

   

A box with a mass of 12 kg is at rest on a horizontal floor.? The coefficient of friction between the box and the floor is 0.7.? A horizontal force of 80 N is applied to the box.? Describe the force of friction in this situation and determine whether or not the box will begin to move.

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