On methods to know the type of friction

Question

Suppose three blocks are stacked, one on the another and a constant force is applied on the bottom one, how could I know if the friction that is acting on those objects is kinetic or static? Moreover, how could I extend this method to N blocks? I understand the method with 2 blocks (You assume that they are one block and then compare the acceleration) but I don't know if it applies to the case of three block or more. For the sake of simplicity, one may assume that all masses are equal.

It depends on the coefficient of static and dynamic friction between surfaces and the external force applied — Shreyansh Pathak, Nov 17 '19 at 12:30
But how does it depend? How do I know what type it is (assuming that maximum static friction > kinetic friction)? — 2 False, Nov 17 '19 at 13:09

score 1 · Answer 1 · answered Nov 16 '21 at 20:05

Friction is a tricky physical phenomenon to analyze. Particularly, determining the type (static versus kinetic) and direction of friction forces is not always a trivial issue. Please see this related answer which provides examples to qualitatively determine the direction of friction forces, which involves understanding the type of friction occurring between two surfaces. We can determine the type and direction of friction force based on the following conclusions reached on studying friction.

The contact force referred to as kinetic friction is applied by one surface on another in the direction opposite to that of the relative velocity of the latter surface w.r.t. the former surface.
The contact force referred to as static friction is applied by one surface on another in the direction opposite to that of the 'impending' relative velocity of the latter surface. The direction of the impending velocity, which is a fictitious quantity, is in the direction of the relative acceleration (w.r.t. the surface applying the force) of the point of contact resulting from the dynamics in which the friction force of interest is fictitiously assumed to be vanished w.r.t. the former surface.

In the case of the phenomenon related to blocks described in the OP, it is necessary to know the direction of relative acceleration of the blocks in order to determine the type of friction force between two block surfaces.

score 0 · Answer 2 · answered Nov 17 '19 at 16:47

Well, I would compare the velocity of each block relative to the other blocks, for the bottom block, I would compare its velocity to the block above it, and if it is moving with respect to the bottom block, then there must be kinetic friction there. The max static friction applied for the top and bottom of block 1 would be different, though, because there is more force acting at the bottom of the first block as opposed to the top of the first block. I would do this same sort of relative analysis with every block, to see if the force applied exceeds the maximum static friction value.

On methods to know the type of friction

2 Answers2