There two main types of friction include
i) Static friction force
ii) Dynamic friction force
Static Friction Force
Static friction force is the friction at rest. This force causes some materials to be stationary Example, book can keep in the top of desk without drops down
From:
Fr = μR
Where:
Fr = static friction force
R = normal reaction
μ = coefficient of static friction force
Coefficient of static friction
Coefficient of static friction is the ratio limiting friction force to the normal reaction.
Dynamic Friction Force
Dynamic friction force is the friction occurs when object moving relative to each other. Dynamic friction force also called kinetic friction force.
From:
Fr = μR
Where:
Fr = kinetic friction force
R = normal reaction
μ = coefficient of kinetic friction force
Coefficient of kinetic friction
Coefficient of kinetic friction is the ratio kinetic friction force to the normal reaction.
Note
1. The coefficient of dynamic friction is less than the coefficient of static friction
2. Usually it requires less force for keeping an object that is already moving in motion that to make it start moving.
Normal Force
Normal force is the equal and opposite to the weight of the body. Always it’s perpendicular to the surface on which it rests
Limiting friction is the minimum force required to move a body over one another
NB:
i) From above diagram: R = mg
ii) when body at rest Fr = F
iii) when body at motion Fr ≠ F so we have to find net force, F
iv) when body start to move static friction force is equal to limiting friction minimum force applied tend to start motion
v) when body start to move kinetic friction force is not equal to minimum force applied tend to start motion
vi) If limiting friction less than force applied the body move
vii) If limiting friction greater than force applied the body cannot move
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