  In many sliding systems consisting of solid object on a solid substrate under
dry condition, the friction force does not depend on the apparent contact area
and is proportional to the loading force. This behaviour is called Amontons'
law and indicates that the friction coefficient, or the ratio of the friction
force to the loading force, is constant. Here, however, using numerical and
analytical methods, we show that Amontons' law breaks down systematically under
certain conditions for an elastic object experiencing a friction force that
locally obeys Amontons' law. The macroscopic static friction coefficient, which
corresponds to the onset of bulk sliding of the object, decreases as pressure
or system length increases. This decrease results from precursor slips before
the onset of bulk sliding, and is consistent with the results of certain
previous experiments. The mechanisms for these behaviours are clarified. These
results will provide new insight into controlling friction.
