When an object is in contact with a rough surface, which is a surface with Friction
coefficients, the object experiences friction forces, these forces are always acting
in the opposite direction of any applied forces:
Every rough surface has a coefficient of friction, this coefficient determines the effect
of the friction force on the object, and there are two types of friction coefficients:
Static Friction coefficient (meu-s):
This coefficient is used to determine the maximum static friction force, which is the maximum Force
that can keep the object static (not in motion) if there exists applied forces on the object,
if the applied forces are greater than the static friction forces, the object is no longer static and will start to move.
Kinetic Friction coefficient (meu-k):
This coefficient is used to determine the maximum kinetic friction force, which is the friction Force
applied on an already moving object.
The Friction Formula:
TO calculate the friction force, the friction coefficient is multiplied by the normal force perpindicular to the surface:
Friction = meu × N
Maximum Static Friction = meu-s × N
Maximum Kinetic Friction = meu-k × N
On a horizontal rough surface:
The weight of the object doesn't contribute to the object's motion due to the
Normal force that is perpindicular to the surface, the weight force and the normal force are equal
and in opposite direction, so they cancel each other out.
If a force is applied on the object parallel to the surface,
and is less than the maximum static friction, the object will NOT move,
and the friction force will be equal to the applied force:
Applied Force = Friction
If a force is applied on the object parallel to the surface, and is equal
to the maximum static friction, the object is about to move, and the applied force is equal to the maximum static friction:
Applied Force = Maximum Static Friction
If a force is applied on the object parallel to the surface, and is greater than the maximum
friction force, the object will be in motion, and the friction will be kinetic.
On an inclined rough surface with angle theta to the horizontal:
The Normal force perpindicular to the surface is equal to the object's weight multiplied by the cosine
of the angle of inclination:
Normal Force = m × g × cos(theta)
The other component of the weight force is acting downwards on the object,
which is the object's weight multiplied by the sine of the angle of inclination,
is considered the applied force:
Downwards Weight Force = m × g × sin(theta)
When there are no applied forces:
The weight component that is acting downwards on the object is the applied force,
and the friction force acts on the object upward the inclined plane.
When there is an applied force acting upwards:
The friction force will act downwards on the object, and the object's weight component that
is parallel to the inclined plane is always acting downwards, the applied force must be larger than
the sum of the friction force and the weight force to pull the object up the inclined plane.
When there is an applied force acting downwards:
The friction force will act upwards on the object, and the weight component that is parallel
to the inclined plane is always acting downwards, to stop the object from moving downwards, the
friction force must be larger than the sum of the applied force and the weight component of the object
Note: the rules of the static friction and kinetic friction on a horizontal rough
surface are also appleid to the inclined rough surface.
