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Some Facts About Friction
What is Friction?
Everyday we experience friction, it is necessary for us to walk or for the wheels on vehicles to turn and move.
So what is friction? How does it work?
Friction is defined as the resistance to motion between contacting surfaces. The effect of friction is dependent on the contacting surfaces and the material properties of the solids in contact.
There are three rules to friction that should be discussed before we examine friction in more depth.
1.) Friction is independent of the surface area in contact.
2.) Friction is directly proportional to the load or the force pushing the surfaces together.
3.) Friction is independent of the sliding speed or the objects velocity.
One would think that the roughness of the surface or the size of the contact area matters in the amount of friction between objects. Rule two helps to explain why rule one is true.
Friction is dependent on the amount of force being exerted on the surface, which increases the surface area which in turn increases the friction.
The amount of force being exerted on the surface plays the major role and the speed or velocity of the object sliding plays a very secondary role.
All of this may seem confusing so let's take a deeper look at friction.
Static Friction and Kinetic Friction
Friction equals the coefficient of friction for the material of the surface in contact times the normal force, which is equal yet opposite with the force of the weight exerted by the object onto the surface.
f = Magnitude of Friction
u = Coefficient of Friction
N = Normal Reaction Force
f = uN
The coefficients of friction for different materials are listed below.
Lets look at N, or normal force, a little closer. The load is the downward force on the object. The normal reaction force of the surface of the material is equal and opposite to the load.
Or the magnitude of the normal force is equal to the magnitude of the force that is pressing the surfaces together.
When force is applied parallel to the surface and no motion occurs the applied force is balanced by an opposite force. This force is called static friction, f(s), and is exerted on the material by the surface that is making contact.
Or, if a block of wood is not moving on an inclined surface the force of the weight exerted equals the force of static friction of the surface of the material the block is in contact with.
The force of static friction is equal to the coefficient of static friction, u(s), of the surface of the material in contact times the normal force.
When the force of the weight exerted by the object exceeds the force of static friction the object moves.
Kinetic friction is the force placed upon an object once motion begins. When the kinetic friction and load is less than the static friction and load, the object will accelerate.
If the kinetic friction is less than the static friction without load, it would take more force to get the object moving.
When a block of wood moves down an inclined plane the coefficient of kinetic friction, u(k), equals the force of kinetic friction, f(k), divided by the normal force, N.
On an inclined plane the force of kinetic friction equals the force of the weight times the Sin of the angle of incline.
The normal force equals the force of the weight times the Cos of the angle of incline.
The forces of weight cancel and the coefficient of kinetic friction can be determined by the Tan of the angle of incline independent of the mass of the object.
Coefficients of Friction
Here is a listing of some materials with their coefficents of static and kinetic friction:
Material u(s) /u(k)
Steel on Steel 0.74 /0.57
Aluminum on Steel 0.61 /0.47
Copper on Steel 0.53/ 0.36
Rubber on Concrete 1.0 /0.8
Wood on Wood 0.25-0.5/ 0.2
Glass on Glass 0.94 /0.4
Waxed Wood on Wet Snow 0.14 /0.1
Ice on Ice 0.1/ 0.03
Teflon on Teflon 0.04 /0.04
Synovial Joints in humans 0.01 /0.003
All the above values are approximations.