# Illusion of static friction: Does it really exist?

13.01.2021

Static friction might be an illusion made by the rotation of kinetic friction, even though, since the time of Charles-Augustin de Coulomb (1736–1806), we have believed that there exist two types of solid friction. Nakano and Popov [1] analyzed numerically and theoretically the nonlinear dynamics of a 2DOF mass-spring system affected by solid friction, considering a non-zero angular misalignment of the spring in the contact plane. This “in-plane misalignment” is the most important piece of the puzzle. According to them, as soon as the contact plane (or the spring) gets pulled laterally, a small in-plane misalignment (e.g., one degree or even less) makes the force vector of “kinetic friction” be “perpendicular” to the pulling direction. After that, the force vector rotates slowly to the “parallel” direction, which generates a “stick” phase followed by a rapid transition to a “slip” phase. This counterintuitive dynamics without “static friction” explains some unsolved frictional phenomena: the slow creep and high-frequency oscillations observed in the “stick” phase. Their finding indicates that the well-known and much-debated properties of the transition from stick to slip may have a completely different – and much simpler – purely mechanical origin.

[1] K. Nakano & V. L. Popov, Dynamic stiction without static friction: The role of friction vector rotation, Physical Review E 102, 063001 (2020), https://journals.aps.org/pre/abstract/10.1103/PhysRevE.102.063001

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#### 1 Comment

1. Jack Huang says:

“Creep” in motion looks like to be originated from the movement of railroad wheelsets.
If your theory of “creep” is from the railroad wheelsets, you can be assured that the theory of “creep” is from imagination not reality, due to the originator did not understand the mechanism of wheelset motion.

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