Doubt in coefficient of friction graph

Question

We all are familiar with the graph of friction coefficient versus time which is as follows:

So I have the doubt here that why the coefficient of friction dips down when the body starts translating motion. Almost every book doesn't try to explain actually how, but rather effect: "as the body starts moving its easy to push forward as friction lowers". This is the explanation given in most high school books I read so far, so, I want to know the actual mechanism, as to why it lowers down. Another explanation given is that molecular interaction lowers down when the body is in motion. This doesn't seem satisfying, because then I thought that at higher velocity/acceleration, the interactive force would lower down even more, but the coefficient remains the same no matter how fast is relative motion. My explanation could be wrong, so I would like everyone to find any faults in it if any.

Pardon me if it seems a very silly/naive question but I can't find any satisfying explanation on the internet also, so please provide any sources if available to you. Thanks in advance.

Answer 1

To begin with, I don't care much for the diagram you obtained as it implies a gradual transition over time between the maximum static friction friction force and the onset of the constant kinetic friction. In my view a better diagram of the friction plot is one relating friction to applied force as found here: http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html#kin

You'll note that the transition between static and kinetic friction is shown as a vertical line, implying a sudden decrease in friction from static to kinetic where the friction force is undefined during the transition. This is consistent with observation. For example, if you try to push a heavy box along on a floor the static friction force prevents movement. When your applied force matches the maximum possible static friction force the box "breaks free" resulting in kinetic friction that providing less resistance than the static friction before breaking free. This is reflected in the friction plot of the link.

As pointed out in the link: "The coefficient (of kinetic friction) is typically less than the coefficient of static friction, reflecting the common experience that it is easier to keep something in motion across a horizontal surface than to start it in motion from rest"

This doesn't seem satisfying, because then I thought that at higher velocity/acceleration, the interactive force would lower down even more, but the coefficient remains the same no matter how fast is relative motion.

I assume you are talking about the kinetic friction force. The coefficient of kinetic friction is generally presented as being independent of the speed of the sliding object. However, that is generally only true for a range of low speed. As stated in the link "when two surfaces are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds"

Bottom line, while we would like to know precisely "the actual mechanism(s)" involved relating friction to surface conditions, speed, and such as you have asked, those mechanisms, to quote the link "defy precise description"

Hope this helps.