1. Physical knowledge:
(1) Understand that uniform circular motion is variable motion;
(2) Grasp the physical meaning and quantitative relationship of linear velocity, angular velocity and period of uniform circular motion;
(3) Master the concept and calculation formula of centripetal force.
2. Through the process of establishing the concepts of uniform circular motion and centripetal force, cultivate students' observation ability, abstract generalization and inductive reasoning ability.
3. Infiltrate scientific method education.
Second, analysis of key points and difficulties
The establishment of the concept of centripetal force and its calculation formula are the key and difficult points in teaching. Through life examples and experiments, we can strengthen our perception and break through difficulties.
Third, teaching AIDS
1. turntable, umbrella;
2. Tie a small ball at one end of the rope (students in pairs);
3. The centripetal force demonstrator.
Fourth, the main teaching process
(A) the introduction of new courses
Demonstration: Throw a chalk head vertically downward, horizontally upward and obliquely upward, and observe the movement track.
Review question: What are the conditions for the chalk head to move in a straight line and a curve?
Inspire students to answer: the direction of speed and the direction of force are in the same straight line, and the object moves along a straight line; Not in a straight line, do a curved motion.
Further question: In curvilinear motion, there is a special form of motion. The trajectory of an object is a circle or an arc (demonstrated by a simple pendulum), which is called circular motion. Please give an example.
(Examples given by students and supplemented by teachers)
When electric fans and windmills rotate, the trajectories of all points on them are circular. From the movement of cosmic objects such as the moon around the earth to the movement of electrons around the nucleus in the microscopic world, they can all be regarded as circular movements. This is a common form of exercise.
Ask a question: why does it lean slightly to the inside of the corner when running 400 meters? Why is the road surface always high on the outside and low on the inside at the corner of railway and expressway and the circular lane of racetrack? It can be seen that the knowledge of circular motion is very useful in practice.
Introduction: In physics, the basic method of studying problems is to start with the simplest situation.
Blackboard writing: uniform circular motion
(B) Teaching process design
Thinking: What kind of circular motion is the simplest?
Guide the students to answer: the speed of an object is constant.
Blackboard: 1. uniform circular motion
The length of an arc that an object passes through at the same time, such as the movement of the needle tip of a mechanical clock.
Thinking: A remarkable feature of uniform Ruta movement is periodicity. What physical quantity can describe the speed of uniform circular motion?
(Students speak freely)
Write on the blackboard: 2. A physical quantity describing the speed of uniform circular motion.
No change.
T is short and S is short, which is the instantaneous speed at a certain moment. Linear speed is actually the instantaneous speed at which an object moves in a circle. When an object moves in a uniform circular motion, the linear velocity is always the same, but the direction is always changing. So, what are the characteristics of linear velocity direction?
Demonstration: pour water on the umbrella and shake the turntable at the same time. Observation: Water drops fly out in the tangential direction.
Thinking: What does this mean?
Teacher-student analysis: at the moment when the flying water drops leave the umbrella, due to inertia, the original speed direction should be maintained, thus indicating the tangential direction, that is, the linear speed direction at this time.
Blackboard: direction: along the tangent direction of each point on the circumference. As shown in figure 3.
Unit: radian/second
(3) Period: the time taken for a particle to move around a circle. For example, the period of revolution of the Earth is about 365 days, and the second hand period of the clock is 60 seconds. , indicating that the action is slow.
(Angular velocity and period can be completed by students themselves and reading books)
Writing on the blackboard: (completed jointly by teachers and students)
Thinking: When an object moves in a uniform circular motion, does V and T change? (,t remains the same, the size of v remains the same, and the direction changes. )
Narrator: Uniform circular motion is the abbreviation of uniform circular motion, which is a variable speed motion.
Put forward the question: uniform circular motion is a curvilinear motion. According to the condition that the object moves in a curve, the object must be acted by the resultant force which is not in a straight line with its speed direction. What are the characteristics of the direction of this resultant force?
Student experiment (in pairs):
One end of the line is tied with a small ball, so that the ball makes a uniform circular motion in the horizontal plane. The weight of the ball is very small (rubber plug can be used instead), and the linear speed of the swing should be as large as possible, and the gravity of the ball can be ignored compared with the pull force, so as to ensure that the pull wire is roughly in the horizontal direction.
Observe and think:
(1) ball force?
② What are the characteristics of the tension direction of the line?
(3) What happens if you disconnect or let go?
Ask the students to write on the blackboard and give examples.
Summary: To make an object move in a uniform circle, it must be subjected to a force perpendicular to the speed direction and pointing to the center of the circle, hence the name centripetal force.
Write on the blackboard: 3. Centripetal force: the force required for an object to make a uniform circular motion.
Ask a question: What factors are related to the magnitude of centripetal force?
(Students imagine doing small experiments with the instruments just now, and experience them roughly by feeling. Students can speak freely after they have their own opinions through experiments and discussions. )
Demonstration experiment (to verify students' ideas): to study the quantitative relationship between centripetal force and object mass m, orbit radius r and angular velocity.
Question: Can the three quantities be changed at the same time in the experiment?
Keep two quantities constant and make a variable.
Experimental device: centripetal force demonstrator.
Demonstration: Shake the handle and the ball will make a uniform circular motion.
Question: What force provides centripetal force? How to measure?
The ball presses the baffle outward, and the reaction force of the baffle to the ball points to the rotating shaft, providing centripetal force for the ball to do uniform circular motion, and the two forces are equal. At the same time, the force of the ball pressing the baffle makes the other end of the baffle compress the spring sleeved on the shaft, and the number of compressed squares of the spring can be read from the scale to show the centripetal force.
Presentation content:
① the relationship between centripetal force and mass: r must be constant, and take two balls to make mA=2mB for observation: (students read) FA=2FB Conclusion: centripetal force Fm.
② Relationship between centripetal force and radius: m, certain. Take two balls for observation: rA=2rB: (students read) FA=2FB Conclusion: centripetal force Fr
③ the relationship between centripetal force and angular velocity: m and r are constants, so that A=2B: (students read) FA=4FB conclusion: centripetal force F2.
Induction: According to the above experimental results, we can know that the centripetal force required for an object to make a uniform circular motion is directly proportional to the square of the mass, radius and angular velocity of the object. But we can't draw a general conclusion from one experiment and one measurement. In fact, we have to do a lot of measurements and experiments, but we can't do them all. The experiment that students have just done shows that the greater M, R and F, the greater F; If the experiment is slightly improved, such as the small experiment introduced in the textbook, and a spring scale is added to measure F, a general conclusion can be drawn (students are required to go back and do it). We can also design many experiments to come to this conclusion, which shows that this is a conclusion with * * *. By measuring the values of m, r and, we can know that the magnitude of centripetal force is F=mr2.
Feedback exercise:
① For an object moving in a uniform circular motion, the following statements are correct: A the speed is constant; B the rate remains unchanged; C angular velocity is constant; The d period is constant.
② As shown in Figure 7, it is a belt transmission device, and the belt does not slip during transmission. Try to compare the linear velocity and angular velocity of a, b and c points on the wheel.
(3) It is said that the relationship between the centripetal force and the radius required for an object to make a uniform circular motion is directly proportional and inversely proportional. How to understand these two statements?
(4) (echo before and after) Explain why the body will lean when running 400m corners. (The train transfer requires reading after class)
Verb (abbreviation of verb) course summary
1. Scientific method
① Point out the process of establishing concepts: summarize and abstract the essential content through a large number of examples, that is, the thinking process from individual to general.
(2) The process of experimental induction is pointed out: after many experiments and enough facts, a general conclusion can be drawn from some special conclusions.
2. Knowledge content: (see blackboard writing)
3. Understanding of centripetal force: centripetal force is not a special force, and its name is only named according to the effect of always pointing to the center of the circle. We will discuss it further next class.
Description of intransitive verbs
1. Centripetal force and centripetal acceleration's teaching sequence. There are two ways to establish the concept of centripetal force: one is to establish the concept of centripetal force through experiments, summarize the formula of centripetal force, and then deduce the centripetal acceleration; Secondly, firstly, the centripetal acceleration is deduced by theoretical deduction, and then the centripetal force is deduced.
Let's talk about acceleration first. The theoretical deduction is rigorous, which can train students' reasoning ability, but the method is abstract and difficult for students with poor foundation. Considering the actual situation of students in my class, I choose to talk about centripetal force first, which reduces the difficulty and makes it easier for students to understand and accept. The current compulsory textbooks also adopt this order. Disadvantages are: because there are errors in the experiment, only a general conclusion can be drawn, it is impossible to do many experiments in class, and the facts summarized in the experiment are insufficient. The key to solve this problem is to minimize experimental errors, supplement examples and make up for the lack of experimental facts.
2. For the teaching of centripetal force, this section has completed the process of perception, generalization and definition, that is, from individual to general and simple cognition. Further cognition is generally personal and left to the next section, so the teaching goal of centripetal force in this section is to master it initially.
Centripetal force excellent teaching plan 2 teaching goal
I. Knowledge and ability
1, know the definition and direction of centripetal force, and know the function and source of centripetal force through examples.
2. Understand what factors are related to the magnitude of centripetal force through experiments, preliminarily master the formula of centripetal force and calculate it.
3. Knowing centripetal acceleration and his formula, we can use Newton's second law to analyze the centripetal force and centripetal acceleration of uniform circular motion.
4. Experience the formation of centripetal force and centripetal acceleration's concept, and boldly express your understanding of related issues.
Second, the process and methods
Through centripetal force theory analysis to experimental exploration, students' literacy and ability to guide practice with theory are cultivated.
Third, emotional attitudes and values
Cultivate students' ability to observe life and think about life phenomena, and at the same time cultivate students' scientific literacy of bold analysis and exploration, as well as the objective materialism spirit of respecting experiments and practice.
Teaching focus
The establishment of the concept of centripetal force and the experimental exploration of centripetal force are the key points of teaching.
Teaching difficulties
It is also difficult to establish the concept of centripetal force and explore the magnitude of centripetal force through experiments. Through simple examples and grouping experiments, we can strengthen our perception and break through the difficulties.
training/teaching aid
1, 16 sets of balls, strings and smooth boards.
2. Small hammer 16 pairs.
3. 16 centripetal force demonstrator.
4. courseware.
teaching process
First, the introduction of new courses.
Watch the video: China's Zhao Hongbo and Shen Xue won the gold medal in the figure skating competition in the 2006 Winter Olympics with wonderful performances, winning glory for their country. What action can Shen Xue's action in the video be approximately regarded as? (Student answer: uniform circular motion), what is the reason why its motion state has been changing? (Student answers: If it is strong, it will produce (acceleration). In this lesson, we will discuss the characteristics of resultant force and acceleration of objects moving in a uniform circle.
Writing on the blackboard: centripetal force and centripetal acceleration