About compulsory physics 2 in senior one Chapter 6 Gravity and space flight

Essentials of solving problems in celestial motion

The movement of celestial bodies is the focus and difficulty of college entrance examination over the years, and it is the concrete embodiment of the application of the law of universal gravitation. The key to break through this difficulty is to know that almost all gravity problems are related to the knowledge of uniform circular motion. Considering the motion of celestial bodies as uniform circular motion, all the centripetal forces needed come from gravity.

Just remember that f = GMM/R2 = MV 2/R = MRW 2 = MR (2π/t) 2 = MA, and choose the appropriate formula for analysis according to the actual situation.

It can be about equal to mg near the earth.

I still remember a gold replacement formula, the topic is when the gravitational acceleration of the central celestial body is used.

GMm/r^2=mg

Main test sites: the changes of acceleration, period and speed with the radius of celestial bodies around the same center.

Just remember that the larger the radius, the smaller everything except the period.

There are also orbital transfer problems, synchronous satellites, the difference between the rotation of objects in equatorial and near-earth orbit, and so on

Reasons for difficulties:

Satellite problem is a comprehensive application of Newton's law of motion, basic law of kinematics, law of conservation of energy, law of universal gravitation and even law of electromagnetism. The reason why it has become one of the difficulties in physics teaching in senior high schools is nothing more than the following reasons.

1, the physical model of the satellite cannot be established correctly, which leads to negative cognitive transfer.

Due to the limitation of high school students' cognitive psychology and the span from Newton's law of motion to the law of celestial motion, they can't distinguish the kinematic characteristics and dynamic characteristics of satellites, spaceships, space stations, space shuttles and other celestial bodies rotating around the earth, and can't establish physical models (including process models and state models) of satellites or celestial bodies moving in a uniform circle. When solving problems, they will naturally be bound by the old kinematics ideas and methods, which will lead to negative cognitive transfer.

2. It is impossible to correctly distinguish the types of satellites, which leads to confusion in understanding.

Satellites can be divided into low-orbit satellites, medium-high orbit satellites, geosynchronous orbit satellites, geostationary orbit satellites, sun-synchronous orbit, large elliptical orbit satellites and polar orbit satellites. According to scientific purposes, it can be divided into meteorological satellites, communication satellites, reconnaissance satellites, scientific satellites, application satellites and technical test satellites. . . . . . Because satellites with different titles correspond to different laws and states, and students' classified names can't correspond to satellite knowledge in textbooks, which leads to mistakes in understanding and application.

3. Incorrect understanding of physical meaning leads to conceptual errors.

There are many terms and concepts in satellite problems, such as satellite, binary star, planet, star and black hole. The moon, the earth, Saturn, Mars and the sun; The orbit radius of the satellite and its own radius; Satellite period of revolution and satellite rotation period; Centripetal acceleration of the satellite, gravitational acceleration of the orbit where the satellite is located, and gravitational acceleration of the earth's surface; Chasing, docking, orbit change, injection, synchronization, launch and orbit entry of satellites. . . . . . Because we don't know the meaning of many concepts involved in satellite questions, mistakes often occur in the process of reading, examining and solving problems.

4, can't correctly analyze the stress, leading to confusion in the application of law.

Because the knowledge of force analysis of objects learned in Senior One is incomplete, the understanding is neglected, and Newton's law of motion, circular law of motion and curvilinear law of motion are unfamiliar or even forgotten, which makes it impossible to transfer and apply these knowledge to the analysis of satellite operation principle and establish correct analysis ideas, leading to the misuse of formulas and laws, and its problem-solving mistakes are inevitable.

5. We can't fully grasp the knowledge system of satellite problems, so that we can't correctly distinguish the differences of nearby knowledge points. For example, Kepler's law of planetary motion is different from the law of universal gravitation; Centripetal acceleration, where equatorial objects revolve around the earth, is different from centripetal acceleration, where synchronous satellites revolve around the earth. The difference between the centripetal acceleration of the moon's motion around the earth and the gravitational acceleration on the moon's orbit; The difference between centripetal acceleration and tangential acceleration of satellite motion around the earth; The difference between the running speed and the launching speed of the satellite; The difference between the three equivalent relationships of gravitation, gravitation and centripetal force; The difference between the radius of celestial body and the radius of satellite orbit; The distance l between two celestial bodies is different from the orbital radius r of one celestial body. . . . . . Only by clearly grasping the differences between these closely related knowledge points can we correctly analyze and solve the satellite problem.

Difficulty breakthrough strategy:

(a) Defining the concept of satellite and the applicable laws;

1, the concept of satellite:

An unmanned or manned spaceship made by humans and launched into space can orbit the earth in space (at least once) for scientific research and application. In the process of studying physics in high school, we should abstract it as an object that can turn around the earth.

2. Applicable law:

Newton's law of motion, law of universal gravitation, Kepler's law of celestial motion, law of conservation of energy, law of circular motion, law of curvilinear motion, law of electromagnetic induction. . . . . Adapted to the satellite problem. However, we must pay attention to the fact that there is a fundamental difference between the stress of a satellite running in the sky and that of a moving object on the ground.

(2) Understanding the classification of satellites:

In the process of physics study in senior high school, it is not necessary to know the specific classification of various satellites and their orbital shapes, as long as the characteristics and differences between geosynchronous satellites (relatively static with the earth) and general satellites (orbiting the earth) are clearly recognized.