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Almost in all diagrams in mechanics, I noticed that a surface is represented by a line with a lot of slashes ("/") on one side, like the one shown below:

enter image description here

I've seen this in optics where such slashed lines are used to denote perfectly reflecting surfaces (mirrors). But what is its significance in mechanics? Before reading the chapter friction, I thought these denote frictionless surfaces, but these slashed lines are even used to show rough surfaces. Is there any difference between a plain line and a line with slashes on one side? Or is that because it looks nice?

I searched various books to find the valid reason for this, but I could not find any. If possible, kindly support your answer with a reference.

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    $\begingroup$ I don't think I've ever seen this used out of India, so it might be useful to ask in the "Physics for Indian exams" chatroom. $\endgroup$ – knzhou Nov 18 at 5:11
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    $\begingroup$ The answer is correct, although it is not so common looking at mechanics graphs. It is implicitly used to show immovable attachment , also to show solids they use a second parallel line to the diagram .. $\endgroup$ – anna v Nov 18 at 5:27
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    $\begingroup$ I've seen it used simply to disambiguate the side that is solid (i.e. part of the depicted object) from the side that is not part of the object. $\endgroup$ – dmckee Nov 18 at 6:53
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It is customary to represent immovable(fixed) objects this way in mechanics. This means that Newton's second law does not apply to it. No matter what are the sum of forces acting on it it will never accelerate (or move at all). Think of the walls or the floor when only small objects and forces are considered.

I don't think I can find a reference on this, but maybe someone can.

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