The transition state in which one physical phenomenon transforms into another is called a critical state. A physical problem in a critical state is called a critical problem. The method of solving critical problems is called the critical method. It is often necessary to solve the problem from the critical situation, and look at the analysis later. There are critical issues in all parts of high school physics.
Like what
First, the critical problem in statics: the critical state of the equilibrium object refers to the state in which the equilibrium state of the object is about to be destroyed but has not yet been destroyed.
The key to solving the critical problem is to find the critical condition. Physical method: Physical method refers to the full use of physical state and physical laws, analysis of critical states or boundary conditions, in a special state, according to the physical laws column equations, you can directly solve the critical problem.
Physical methods include (1) utilizing critical conditions, (2) utilizing boundary conditions, and (3) utilizing vector graphics.
Critical problems are associated with extreme value problems, and the main difference is that critical problems are usually physical, and extreme value problems are usually mathematical. For example, look at the picture below.
Another example,
Critical problems in dynamics
Critical problems in dynamics, critical conditions are mainly the following:
(1) Critical conditions for contact and disengagement: elasticity between two objects
(2) Critical conditions for relative sliding: the static friction force reaches the maximum
(3) The critical condition of rope break and relaxation: fracture: the tension in the rope is equal to the maximum tension it can withstand, relaxation:
(4) The critical condition of maximum acceleration and maximum speed: Under the action of changing external forces, the acceleration of the object is the largest when the external force is the largest, and the acceleration of the smallest external force is the smallest; when the acceleration is 0, the speed is often the largest.
Critical problems in circular motion
(1) The critical problem of circular motion on the horizontal plane
The object is placed on a rotating disk, and together with the disk, it makes a uniform circular motion, and the static friction force passes through the centripetal force.
The critical condition that the object does not happen to slide relative to the disk is: the maximum static friction force just provides the centripetal force,
This is the critical angular velocity. When the disk rotates at an angular velocity, the object will do a centrifugal motion.
Example 4: The problem of the linkage wheel friction providing centripetal force
(2) Critical problem of circular motion in the vertical plane
The light rope model and the light bar model are subjected to force criticality at the highest point, and the minimum centripetal force at the highest point of the rope object is the gravity of the object, while the minimum centripetal force at the highest point of the pole model is 0.
Critical problems in electromagnetic induction
There are often cases where the rod moves to the final uniform speed, and the equilibrium equation is used to solve it. Or the rod changes to slow down the movement to finally stop. and so on. Energy methods are often used. The kinetic energy theorem is commonly used. The kinetic energy theorem is consistent with conservation of energy. In the kinetic energy theorem, the conversion of energy is reflected by the work, in which overcoming ampere force work is equivalent to the conversion of mechanical energy into electrical energy and then into internal energy, that is, Joule heating.
The problem of "Joule fever" in the "electromagnetic induction" question is also a problem that often occurs in the college entrance examination questions.
The so-called "Joule heating" is the heat generated by the current, and the "Joule heating" in "electromagnetic induction" is the heat generated by the induction current. There are usually 3 ways to find "Joule fever":
First, the direct method, which is solved according to the formula;
The second is the indirect method, which is solved by applying the kinetic energy theorem or the law of conservation of energy.
The third is to solve the relationship between work and power. The second method is used for this question.
Also note whether the topic is the heat generated on the resistor R, or the total joule heat of the loop.
In the question of energy conversion and conservation related to electromagnetic induction, to clarify the relationship between what force does work and what energy is transformed, they are:
Combined effort = change of kinetic energy;
Gravity does work = change of gravitational potential energy; gravity does positive work, gravity potential energy decreases; gravity does negative work, gravity potential energy increases;
Elastic work = change of elastic potential energy; elastic force does positive work, elastic potential energy decreases; elastic force does negative work, elastic potential energy increases;
Electric field force work = change of electric potential energy; electric field force does positive work, electric potential energy decreases; electric field force does negative work, electric potential energy increases;
Ampere force does work = the change of electrical energy, ampere force does positive work, and electrical energy is converted into other forms of energy; Ampere force does negative work (i.e. overcome ampere force to do work), and other forms of energy are converted into electrical energy.
Wait, for detailed analysis, please see the picture
