Definition of critical value
The critical value refers to the condition that a certain physical quantity must satisfy when the object changes from one physical state to another physical state, which is equivalent to the stagnation point often said in mathematics.  Therefore, using the critical state to solve the maximum and minimum values of physical quantities has become an important method for solving the maximum value in physics. Some people think that using the critical state to solve the maximum value should be cautious. First, we must distinguish the relationship between the two states.