between it and the surface it’s on is 0.5. Then, the maximum force that static
friction can exert is (0.5)(100 N) = 50 N. However, if you push on the object with a
force of, say, 20 N, then the static friction force will be 20 N (in the opposite
direction), not 50 N: The object won’t move. The net force on a stationary object
must be zero. Static friction can take on all values, up to a certain maximum, and
you must overcome the maximum static friction force to get the object to slide.
Kinetic vs. Static
For a person walking, the
friction between the
person’s shoes and the
floor is static (no sliding)
and is directed forward
(in the direction the
person is walking). The
person pushes on the
floor in the backward
direction. Static friction
prevents it from moving
backward, and
therefore it must be
forward. For similar
reasons, objects that are
rolling without slipping—
rolling normally, not
skidding—roll because
of static friction.The direction of Fkinetic friction = Ff (kinetic) is opposite to that of motion
(sliding), and the direction of Fstatic friction = Ff (static) is opposite to that
of the intended motion.