Friction Main Page

Friction Forces

The forces of friction are very important to life on our planet.  Friction can cause a  resistance to motion and friction can also be an aid to motion.  Friction occurs when an object is in contact with another object or when an object is moving through another substance.  A football encounters friction from the air as it moves through the air.  Since this friction opposes motion it will cause the football to slow down.  Friction is the force that holds a nut onto a bolt.  Friction between your feet and the ground allows you to walk. It is difficult to accelerate on ice because the force of friction is very low.

Coefficients of Friction

There are two coefficients of friction.  They are the coefficient of static friction and the coefficient of kinetic friction.

The coefficients of friction are dimensionless.

The coefficient of friction between one pair of surfaces will be different from that between another pair of surfaces, depending upon the material that are in contact. The coefficients of friction vary depending upon the material.

Coefficient of Static Friction 

The symbol used for the coefficient of static friction is 'μs'.

The coefficient of static friction is a measure of the resistance needed to get the object to start moving.  It is greater than the coefficient of kinetic friction. You should remember that objects at rest tend to remain at rest so this tendency must be overcome to get an object into motion.

Coefficient of Kinetic Friction 

The symbol used for the coefficient of kinetic friction is 'μk'.

The coefficient of kinetic friction is a measure of the resistance needed to keep an object moving once it is moving.  This is lower than the value of the coefficient of static friction

Force of Static Friction 

The force of static friction between two surfaces is fs = μs

where  fs = force of static friction,  μs is the coefficient of static friction, and n is the normal force that presses the two surfaces together.

Force of Kinetic Friction 

The force of kinetic friction between two surfaces is fk = μk

where  fk = force of static friction,  μk is the coefficient of kinetic friction, and n is the normal force that presses the two surfaces together.

 

Terminal Speed or Velocity

When an object is falling through the air it encounters friction caused by the effects of the air on the object.  As long as the force of friction on the object is less than the force due to gravity, the object will undergo acceleration.  The force of friction will increase as the object accelerates until the force of friction is the same as the force causing it to accelerate in the downward direction.  Once the two forces are equal, but in opposite directions, the acceleration will cease because the net force acting on the object is now zero.  With no net force there can be no additional acceleration so the maximum velocity has been reached.  This maximum velocity is called the terminal velocity.  If you fall out of an airplane with no parachute you will probably hit the ground at a speed of less than 150 miles per hour since by that time you will have reached the terminal velocity.