Friction / Friction definition: If we either slide or attempt to slide a body over a surface, the motion is resisted by a bonding (cold-weld) between the body and the surface.
The resistance is considered to be a force called either the frictional force or simply friction.
This force is directed along the surface, opposite to the direction of the intended motion.
Friction caused by contact forces – how? – friction definition
The following diagram will help us to understand how contact forces between two surfaces give birth to friction.
See, there is a block on a surface.
If we intend to move the block towards left from right, then the block will exert a downward push (towards SouthWest in this case) on the surface which makes an angle with surface.
Now the surface will also apply an equal and opposite force on the block following Newton’s 3rd law of motion.
We have shown the reaction force as F (contact force in the diagram acting upwards towards North East).
Now if we resolve F along the vertical and horizontal direction, then we find the horizontal component to act opposite to the direction of the intended motion of the block.
So obviously this force is resistive to the intended motion of the block and hence we call it Friction or Frictional force. (labelled in the diagram above)
Relative motion of the surface below and friction
Now as the block tends to move here from right to left, the surface below again tends to have a relative motion from left to right.
So will that relative motion will also face a resistive friction?
How? See here.
Now again we can resolve the downward force exerted on the surface by the block.
(This downward force is acting downwards towards SW, we mentioned about this one in previous section as well)
After resolving we see that, the horizontally resolved component is directing towards left.
This is opposite to the tentative relative motion of the surface.
So this is again a frictional force. But this time it acts on the surface below to resist its relative motion.
What we covered here: This topic answers queries like definition of friction, what is friction, how friction happens between 2 surfaces etc