Final answer:
In frost wedging, the principal force is exerted against the walls of the confining rock, caused by the expansion of water when it freezes within the rock's cracks.
Step-by-step explanation:
In frost wedging, the principal force is exerted against the walls of the confining rock. This process occurs when water enters cracks or microscopic openings in rocks and then freezes. The expansion of water upon freezing exerts pressure against the walls of the crack, causing the rock to split and eventually break apart. This is a type of physical weathering that is especially common in climates where temperatures fluctuate above and below freezing.