For example, in the proof of solid geometry in mathematics, many problems use projection method.

In physics, when an inextensible rod or rope rotates around a point, although the speed of each point is different, the projection of the speed of each point along the direction of the rope is the same.

Personal understanding: the so-called inextensibility can be said to be a rigid body. Because it is inextensible, the speed of point A on the rod cannot be different from that of point B on the rod in the direction of the rod (both va 1 and vb 1 should be along the rod), because if they are different, the displacement will increase, that is, the rod or rope will be elongated, which is inconsistent with inextensibility. This theorem is based on this, but we should know that the velocities of point A and point B are not always along the direction of the bar, but according to the (orthogonal) decomposition of vectors. va and vb can decompose va 1 ... respectively along the bar va2 perpendicular to the bar, but the velocities along the bar must be the same, that is, va 1=vb 1, so we can use this theorem to establish the equation.