How to calculate the work done by friction

okay so we have the formula W=F.d
"d" is the change in distance of the contact point of the exerted force and the object
for example if a person is pushing a block, the contact point is between the guys hand and the box.
so d is the change of distance of that point.
but my instructor said that when you have friction its different. considering a block accelerating along a horizontal surface, if yOu zoom in at a microscopic level at the floor and bottom of block, there are jagged lines. so we never know where the point of contact of the friction force is. because its constantly changing. okay so now what. how the hell am i supposed to calculate the friction force. with what formula? HE GIVES US HW ON FRICTION WHEN WE DONT KNOW WHAT FORMULA TO USE. WHAT IS THIS?!

Ok, so sounds like your instructor is trying to confuse you. Just stick to the definition. You know the direction of friction is always opposing a force, so it will be negative. Then don't worry about the "microscopic" level. Just take the friction force multiplied by the distance. W = F*d will give you what you need; just make sure if the person is pushing at an angle, you take that into account by using cosϑ.

The word "point" is misleading, as a point has no dimensions..., read "does not exist".
All you need is the overall average friction between 2 surfaces.
Friction is simply the resistance force to getting something initially moving (the static friction), or the resistance force to keeping it moving (dynamic friction).
Dynamic is lower than static force, for the same surfaces.
Work = (force x distance), so work done by friction will be dynamic friction, and the distance is the distance the object moves.
You need to be given either a force of friction, or the coefficient of friction with which to work out the force.

You use the static or kinetic coefficient of friction :
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F = ( mu sub k ) ( W )

WK = ( mu sub k ) ( W ) ( d ) <-----------------------------------