Which of the following best demonstrates the effect of static friction? A. A person dropping a ball to the ground. B. A person pushing a couch and it slowly sliding across a floor C. A person pushing a couch without being able to move it D. A person moving a ball through a stream of water

C.

Explanation:

A person pushing a couch will face resistive force of friction. When resistive force is greater then his force of effort, couch will not move. This force is static friction because the couch is stationary. When the force of effort is increased, magnitude of static friction also increases keeping the couch stationary. The ability of the static friction to increase its magnitude is limited by a maximum value beyond which the couch starts moving. The static friction is then converted into kinetic friction.

In rest of the three cases object is already moving so kinetic friction is in action.