Why does gravity break down into two forces: vertical rope and along the rope? In high school physics, does looking at the direction of resultant force mean looking at the motion trend of objects at the next moment? The theme of mechanics is to study the relationship between force and motion, so it should be combined with motion when analyzing force.

Because the rope can't be stretched, after cutting L2, the ball A will make a circular motion with a radius of L 1. In order to use the centripetal force formula of circular motion, it is necessary to establish a coordinate system along the tangent direction of the arc and the direction pointing to the center of the circle. After establishing this coordinate system, gravity must be decomposed along the rope direction (pointing to the center of the circle) and perpendicular to the rope direction (tangent direction of the arc), so that the centripetal force equation can be listed: t-mg cos θ = /L 1=0 (at the moment of cutting L2, the speed of ball A is 0).