Kinetic energy theorem: the work done by external force is equal to the change of kinetic energy of an object. Kinetic energy theorem is widely used to find the magnitude of work done by variable force and the speed of final or initial state. In middle school, there is no limit to the use of kinetic energy theorem.

Momentum theorem: If an object is not subjected to external force or the vector sum of external force is 0, the momentum of the object is conserved. The use of the momentum theorem is conditional, that is, the resultant force acting on an object (which can be one object or a whole composed of multiple objects) is zero, so the momentum theorem can be used. It is mainly used to calculate the initial and final velocity of objects when they explode or collide.