Help Clarify Friction: Can Tangential Force be Greater than Frictional Force?

[vappole]

I don't have any good physics books, could someone please help clarify this confusion on friction?

From what I read, I take it that the force of friction, which is proportional to the normal force, is the force keeping the two bodies stuck together. In order to slide one body against the other, this force has to be overcome. Let's call the over-coming force 'tangential force'. The tangential force is thus in the direction of motion in the case of sliding, whereas the frictional force is in the opposite direction.

My question is, is there any reason why this tangential force cannot be greater in magnitude than the frictional force? If so, WHY? This has been puzzling me for days, as engineering books seem to claim this fact, but with no basis, and it doesn't make any sense to me, i.e. I don't see any reason why I should not be able to accelerate one body with respect to the other in the sliding direction.

Thanks!

 

[G01]

So, you have the two forces on a sliding object, let's call them T (tangential) and f(friction) for short.

You are correct, there is no reason why T cannot be greater than f. In fact, this is the only way that the object could increase its velocity.

However, if, as I suspect, your engineering text is assuming that the object is not accelerating, then the two forces must be equal and opposite, because that is the only way $\Sigma F$ and thus the acceleration, a, can be 0.

 

[AlephZero]

Most "elementary" texts don't tell you that what you are talking about is a simple mathematical model of how friction forces work. It was first proposed by Coulomb (the same guy who gave his name to the unit of electrical charge). It is a pretty good model for "hard" materials, moderate levels of force or pressure, and fairly large movements at low velocities. In other words, it works well for the sort of lab experiments that you do in beginning physics courses.

In other situations (e.g highly polished surfaces, "soft" materials like rubber, situations where the friction force generates a lot of heat, etc) it is a very poor model.

Don't worry too much about why it is true, because if you go further into the subject (which you are unlikely to do below postgrad level) you will find that actually it isn't "true" except as a limiting case of some rather complicated physics.

It is the only friction model you will meet at an "elementary" level, because it is the only one that is simple enough to be useful for hand calculations with. Unfortunately, textbooks often call it "Coulomb's law of friction" rather than "the Coulomb model of friction", and students can get the idea that it really is a "law of physics" in the same sense as the ideal gas laws or Newton's laws of motion. That is not the case.