  Amontons' law states that the maximum static friction force on a solid object
is proportional to the loading force and is independent of the apparent contact
area. This law indicates that the static friction coefficient does not depend
on the external pressure or object shape. Here, we numerically investigate the
sliding motion of a 3D viscoelastic block on a rigid substrate using the finite
element method (FEM). The macroscopic static friction coefficient decreases
with an increase in the external pressure, length, or width of the object,
which contradicts Amontons' law. Precursor slip occurs in the 2D interface
between the block and substrate before bulk sliding. The decrease in the
macroscopic static friction coefficient is scaled by the critical area of the
precursor slip before bulk sliding. A theoretical analysis of the simplified
models reveals that bulk sliding results from the instability of the
quasi-static precursor slip caused by velocity-weakening local friction. We
also show that the critical slip area determines the macroscopic static
friction coefficient, which explains the results of the FEM simulation.
