Explain: "Static friction force opposes impending motion".

(N/A) Consider a box lying on the floor of a train compartment.
When the train accelerates, the floor of the train tends to move forward relative to the box.
Due to inertia, the box tends to remain in its original position, which is the 'impending motion' relative to the floor.
Static friction $(f_{s})$ acts between the box and the floor in the forward direction (the same direction as the train's acceleration).
This force provides the necessary acceleration to the box to keep it stationary relative to the train.
If there were no friction, the floor would slide forward, and the box would appear to move backward relative to the compartment.
Since the box moves with the train without sliding, the static friction force effectively opposes this relative sliding tendency (impending motion).
Thus, static friction acts to prevent the relative motion that would occur if the surfaces were frictionless.