Dry Friction Questions: Exploring Forces and Bumpiness

[TheTauist]

So I have been told that friction is caused by microscopic bumpiness in the surfaces of objects. Based on Friction= (coefficient of friction)x(normal force), if normal force is 0, then there is no friction. However, where is the line drawn between microscopic bumpiness causing friction and other things. Why can't the force of a nail holding up a picture not be considered friction? Or can it? Based on the above formula, the friction would be 0 though since the is no normal force perpendicular to the direction of friction. Is the formula a simplified version that is only accurate to a certain level of bumpiness and afterwards, its no longer considered friction? Also why is dynamic friction the same for all speed? Or is it? From my understanding, dynamic friction breaks bumps easier (due to more momentum) and/or does not all the to surfaces to come all the way into each other.

Thanks!

 

[AlephZero]

You are right that Coulomb's "law" of friction is only a simple approximation to reality. Rather than calling it a law, it is better to call it a model of friction (using "model" in the sense of a "mathematical model".) For hard objects, with moderate size forces between them, moving at fairly low relative speeds, it matches "real life" quite well. In other situations it can be very inaccurate. But "better" models of friction are usualy too complicated to use for hand calculations and need computer simulatons to use them, and therefore they are not very useful for teachng people the basics of friction. Unfortunately, some students get the wrong idea the the Coulomb "law" of friction is the whole strory, and it is a "law of physics" in the same sense as Newton's laws of motion of the ideal gas laws.

But in your example, the nail is embedded in something flexible (e.g. a piece of wood) and there is a normal force acting over all over its surface, because the wood is trying to "spring back" to fill up the space where the nail is. To pull the nail out, you have to overcome the friction force created by that normal force.

If the nail is horizontal with a picture hanging from it, there is also a vertical force acting upwards on the nail, equal to the weight of the picture, and that is another "normal force" acting between the nail and the wall.

 

[chrisbaird]

By the way, it's not just "microscopic bumpiness" that causes dry friction, but also molecular attraction due to the Van der Waals force. That is why very smooth surfaces can have surprisingly high friction coefficients.

 

[TheTauist]

So what if the nail we by the wall just having a protrusion large enough to hold something up?

 

[PJC01]

As a scientist, it is important to first clarify that the equation Friction= (coefficient of friction)x(normal force) is an oversimplified representation of friction and does not take into account all factors that contribute to it. Friction is a complex phenomenon that involves many different forces and interactions at the microscopic level.

While it is true that microscopic bumpiness on surfaces can contribute to friction, it is not the only factor. Other factors such as adhesion, surface roughness, and intermolecular forces also play a role in determining the amount of friction between two surfaces. Additionally, the force of a nail holding up a picture cannot be considered friction because it is a different type of force known as normal force, which is perpendicular to the direction of friction.

The equation you mentioned is a simplified version that is only accurate to a certain level of bumpiness. As we delve deeper into the microscopic level, we find that there are many other factors at play that can affect the amount of friction between two surfaces. Therefore, the equation should be used with caution and is not always applicable in all situations.

In regards to your question about dynamic friction, it is not always the same for all speeds. The amount of dynamic friction can vary depending on the speed, surface roughness, and other factors. However, for most practical purposes, it is assumed to be constant at low speeds. At higher speeds, other factors such as air resistance and temperature can also affect the amount of friction.

Finally, it is important to note that friction is a crucial force in our everyday lives and plays a significant role in many natural and man-made processes. As scientists, it is our responsibility to continue studying and understanding friction at a deeper level to fully comprehend its complexities and potential applications.