In the physics questions of the college entrance examination, there is often a hidden condition, although it is necessary for solving the problem, there is no direct text description, cleverly hidden behind the problem, it is not easy to be detected by the solver, and it is often the key to solving the problem, suggesting the method and way of solving the problem, and also determines the correctness of the answer.
Therefore, when understanding the problem, fully explore and use the implicit conditions in the proposition to improve the completeness and accuracy of the solution.
The implicit form of a condition in a physics question usually manifests itself in the following ways:
There are many noun terms in physics, often implying a constant. Such as: standard atmospheric pressure; voltage of the lighting circuit; density and specific heat capacity of water; speed of light and sound in air; g=9.8m/s2, etc.
These data are generally not given in the question, requiring the solver to directly apply it from his own memory.
Such as the mass of the object, the density of the substance, the specific heat capacity of the substance, the resistance of the conductor, the calorific value of the fuel, the voltage of the power supply, the nameplate mark on the electrical appliances, etc.
The conditions of the problem must reflect a number of physical phenomena, which themselves contain the known conditions required to understand the problem. For example, "astronaut in a running spacecraft" indicates that the astronaut is in a weightless state; "communication satellite" indicates that the angular velocity or period of satellite operation is the same as that of the earth, that is, synchronization; "conductor in equilibrium" indicates that the object is an isomorphic body, and the internal field strength is zero.
Idealized conditions are often implicit in the relevant words or meanings in exercises, and ideal models need to be used to capture and excavate. Such as particles and point charges, do not count their shape and size; lightweight springs do not count their weight; smooth surfaces do not count their friction; ideal transformers do not count power losses
Although the critical and extreme value issues in high school physics are not explicitly mentioned in the syllabus or examination instructions, they have appeared frequently in the high school examination questions in recent years.
Judging from the previous form of test questions, some words such as "exactly", "maximum", "at least", "not colliding", "not disengaged" and other words directly in the stem give a clear hint to the critical state. There are also some critical questions that do not contain the above common "critical terms", which have a certain degree of concealment, the flexibility of problem solving is large, and the physical situation of the exercise should be restored when reviewing the question, and the change of state should be carefully discussed.
Judging from the content of the previous test questions, the examination of physical critical problems is mainly focused on the relationship between force and motion, and the examination of extreme value problems is mainly concentrated in the parts with large weights such as mechanics or electricity.
1. The rainwater flows from a smooth sloped roof with a certain horizontal length for the shortest time - the roof inclination angle is 45°.
2. The object thrown from a point on the long slope is the farthest from the bevel - the speed is parallel to the slope at the moment.
3. The object can slide at exactly the same speed along the fixed bevel at the initial velocity (the object does not slide down when it rushes to the fixed bevel) - μ = tgθ.
4. The object just slides - the static friction reaches the maximum.
5. The maximum (minimum) distance between the two objects moving in the same direction - the speed of the two objects is equal.
6. The maximum (minimum) relative velocity of two objects moving in the same direction - the acceleration of the two objects is equal.
7. Displacement of a certain first start and then brake segmented movement, in the initial, final speed and two stages of acceleration at a certain time to make the whole time shortest - there is no uniform speed section in the middle (displacement of a certain first start after the brake segment uniform variable speed movement, in the initial speed and two stages of acceleration a certain time to make the power action time is the shortest - to the end of the end of the time is exactly zero)
8. The two cars do not collide - the two cars are exactly equal speed when the rear car catches up with the front car.
9. The speed of the accelerated moving object reaches the maximum speed - the speed at which it is no longer accelerating.
10. Two objects in contact are just separated - two objects are in contact but the elastic force is exactly zero.
11. The farthest point that an object can reach - the speed of a linear moving object reaches this point when it reaches this point (the trajectory of the curved moving object is exactly tangent to a certain boundary line)
12. The lowest position where you want to hit the ball horizontally above the 3-meter line on the volleyball court - both touching the net and pressing the boundary
13. The object on the board or conveyor belt does not slip off - the object reaches the end at the same speed.
14. The line (rod) end is done in the vertical straight plane to do the circumferential movement just enough to reach the highest point of the circumference - the highest point rope tension is zero (=0v pole end)
15. The non-constrained object moving on the vertical surface reaches the highest point - the vertical and straight division speed is zero.
16. The thin line is just straightened - the thin line is straight and the tension is zero.
17. In the decomposition problem where the direction of one force and the magnitude of the other force is known, if the second force is just very small - the two-part force is perpendicular.
18. In the "two-variable and one constant" three-force model of dynamic force analysis, the "double variable force" is extremely small - the two variable forces are vertical.
19. If the object is to move along a straight line with 1f at an acute angle under the action of two forces of 1f2f, it is known that 1f is a fixed value, then 2f is the most
Hours is exactly the partial force of 1f in the vertical velocity direction.
20. The shortest time to cross the river - the speed of the boat is perpendicular to the river bank, that is, the speed of the boat is perpendicular to the river bank (equivalent to crossing the river in still water).
21. The shortest voyage in the river crossing with a speed greater than the water speed - "oblique reverse sailing" and the speed of the boat is offset by the speed of the water speed.
22. The shortest voyage in the river crossing with a boat speed less than the water speed - "oblique reverse sailing" and the speed of the boat is perpendicular to the combined speed.
23. The "cylinder" rolls up the steps with the least effort - making the power arm reach a maximum of 2r.
24. Collision with the least (large) loss of kinetic energy - fully elastic (completely inelastic) collision.
25. The simple harmonic movement speed is the largest - the displacement (resilience, acceleration) is zero.
26. The amplitude of the forced vibration is exactly the maximum - the frequency of the driving force is equal to the natural frequency of the vibration system.
27. Only when mechanical energy and electric potential energy are converted into each other, the sum of gravitational potential energy and electric potential energy is the smallest, and the kinetic energy is the largest.
28. Particles do not fly out of a uniform magnetic field - the circular trajectory is tangent to the boundary of the magnetic field.
29. The output power of the power supply is the largest when the pure resistance load is equal - the internal and external resistance values are equal.
30. The resistance value of the sliding varistor symmetrical connection method reaches the maximum - slides to the midpoint.
31. When the energized conductor rod on the smooth rail placed obliquely is stationary, the magnetic intensity is minimal if the direction of the uniform magnetic field is perpendicular to the slope.
32. When light is shot from the medium to the air, it does not shoot out - the angle of incidence is equal to the critical angle.
33. Just happen to occur the photoelectric effect - the incident light frequency is equal to the limit frequency.
34. Charged particles happen to be selected by the velocity selector (Hall effect, plasma power generation) - the electric field force is balanced with the Lo force.
35. When the "ground satellite" (hydrogen atom is in the ground state), the potential energy is the smallest, the total energy is the smallest, the motion period and the angular velocity are the smallest; the speed, centripetal force and acceleration are the largest.
![Douluo Continent: Ma Hongjun did not look accustomed to it after changing the mold, and the critical state of change was very pleasing to the eye[fig]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)