Why does the normal force affect friction?

[jumbogala]

Homework Statement

I am student teaching a class about static friction soon.

I am worried a student is going to ask me where F_{static friction} = μ
_sF_normal comes from. I am not sure how to explain it.

Homework Equations

The Attempt at a Solution

I've always thought of it this way. Consider a rock on a table. Normal force is a measure of how hard the table pushes up on the rock.

If the normal force pushes harder, the rough surface irregularities on the table "catch" more on the rock and that increases friction.

But it just occurred to me that I have NO clue if this is true. Can anyone help me out?

 

[Simon Bridge]

Teach friction empirically and students will discover that it is proportional to the weight of the object.

It is commonly taught that friction comes from irregularities in the surfaces that catch on each other - but that is not the only source. See:
http://van.physics.illinois.edu/qa/listing.php?id=18391

If you use the "roughness" model, to slide, the surfaces have to lift slightly.
The harder you press the surfaces together, the harder it is for the surfaces to jolt apart enough to slide, thus, the more friction. The "normal force" is one way of quantifying how hard the surfaces are pressed together.

 

[Astrum]

Well, since nobody bit, I'll try to give my two cents. I'm really just a first year student, so I may be wrong.

I think that if you think of it on the molecular level, it'll make more sense. If you visualize all little crags/valleys, you can see how, if you apply more downward force, you will need more force in the x direction to move it. It would be similar to rolling a wheel over a step.

It makes sens to me, not sure it it really works like that.

It's such an intuitive idea that I doubt you'll be asked about it, I didn't even think of this question until I read your post.