Friction due to surface roughness always opposes relative motion or tendency of relative motion between bodies in contact. Whenever friction acts it always acts on the two bodies in opposite direction. But friction is different when bodies are at rest and tend to slide or these are sliding.
So lets
first discuss about the basics of friction and its classification in the below videos. Friction and its Classification – https://youtu.be/pzijTUmAe2c
Depending upon the state of motion there are three ways in which Friction is considered – Static
friction, Limiting Friction and Kinetic friction. See this video to understand these in detail –
Common mistake of students in Problems of friction: While solving problems involving friction, you
need to be very careful in drawing free body diagrams of bodies as friction always acts on two bodies in
contact in opposite directions. When students draw FBD of sliding bodies in the given situation of
problem, they draw an arrow representing friction on one body and miss it on another body as
psychologically they feel they have considered friction somewhere.
So always remember to draw friction in FBD twice. Both on different bodies in their independent FBDs in
opposite direction.
How we use the concept of friction in various problems using free body diagram(FBD), see the examples
explained in below videos –