I. Knowledge and skills

1. The expression of momentum theorem can be derived from Newton's laws of motion and kinematic formulas.

2. Understand the exact meaning of the momentum theorem and know that the momentum theorem is applicable to variable forces.

3. Will use momentum theorem to explain related phenomena and deal with related problems.

Second, the process and methods

1. Introduce topics through demonstration experiments to stimulate students' interest in learning.

2. Through the process of exploring momentum theorem, try to study physical problems by scientific inquiry, and get the methods and steps to solve momentum theorem through the analysis and explanation of examples.

3. Be able to apply momentum theorem to deal with some practical problems related to production and life, cultivate students' ability to integrate theory with practice, and cultivate communication and cooperation ability in the process of analyzing and solving problems.

Third, emotional attitudes and values

Have the enthusiasm to participate in scientific and technological activities, and have the consciousness of moving from life to physics and from physics to society.

Teaching focus

Deduction of Momentum Theorem and Explanation of Momentum Theorem to Related Phenomena

Teaching difficulties

How to correctly understand that the impulse of the resultant force is equal to the change of the momentum of the object; How to correctly apply momentum theorem to analyze short-term mechanical problems such as impact and collision?

teaching process

First, ask questions and introduce new courses (create experimental scenes)

Problem one

Demonstration: spread a sea mat on the floor, hold the eggs as high as possible, then let go of your hand and let the eggs fall on the sea mat.

First, let the students guess the possible phenomena.

Actual operation: it was observed that the egg was not broken.

Introduction: the egg fell from a height of more than one meter to the sea mat, but it didn't break. Why? We will learn this knowledge in this class.

Question 2 (Situation suggestion creates problem situation)

We learned last class that we can study the influence of a moving object on the outside world through a new physical quantity: p=mv. At a certain moment, an object has a velocity corresponding to a momentum. If the speed of an object changes, its momentum will also change: = p'-p = mv'-mv. So, are we going to ask: Why does the momentum of a moving object change? What is the law of this change? This is what we will learn in this class today.

Question 3 (Life experience creates problem situations)

Automobile braking (by bus, we have this experience) has the same momentum change () during parking, and slowly slides until the parking resistance is small and the action time is long; The sudden braking resistance is large and the action time is short. What is the quantitative relationship between them?

Second, the new curriculum teaching

(1) Guide students to deduce momentum theorem and understand its characteristics.

[Problem] The initial velocity of an object with a mass of m is V, and the velocity becomes V' after a period of time under the action of the resultant force F. It is found that: what are the initial momentum p and the final momentum P' of an object respectively? The acceleration of an object a=?

Derived from Newton's second law: f = ma and a = (v-v)/t.

That is, f equals t = mv-mv = p`-p.

[Discussion] What are the physical quantities in this expression?

【 Conclusion 】 The impulse of the combined external force to the object is equal to the change of the momentum of the object. This conclusion is called momentum theorem.

Understand; Understanding

F-sum in the formula refers to the resultant force of all external forces, including gravity. It can be a constant force or a variable force. When the resultant force is variable force, F-sum should be the average value of variable force during the action time.

This formula is a vector formula and should be applied in the positive direction.

(B) the application of momentum theorem to explain the phenomenon in life (teachers and students * * * with induction, students for example)

The first category: under the condition of certain momentum change, if it is necessary to increase the acting force, the acting time must be shortened, and the common situation is "playing hardball".

For example: hammering nails, punching steel plates, eggs hitting rocks, etc.

The second category: under the condition of certain momentum change, if it is necessary to reduce the acting force, the acting time must be extended, and the common situation is "buffering".

For example, gymnasts bend their lower limbs when landing, seat belts on car seats, bungee cords and eggs fall on sea mats.

The third category: when the external force is constant, the longer the force action time, the more obvious the change of the object's motion state, and the shorter the force action time, the less obvious the change of the object's motion state.

For example, the paper strip under the wooden block at the edge of the table has two different drainage speeds, and the horizontal distance between the wooden block and the edge of the table is also different (fast is close, slow is far).

(c) Through examples, let students summarize the steps of solving problems by using momentum theorem.

1. Determine the research object and movement process.

2. Analyze the force and motion state of the object, and find out the resultant force and initial velocity of the object.

3. Select the positive direction to display the impulse, initial momentum and final momentum of the resultant force.

4. Column calculation.

(4) Consolidate exercises

Students list phenomena in life that can be explained by momentum theorem.

Students' formulaic calculation (first analyze the problem-solving ideas and then do the problems)

A construction worker with a mass of 60 accidentally fell from a height, and he was hung up because of the protection of a safety belt tied with an elastic rope around his waist. It is known that the elongation of elastic seat belt is relatively small, the length of seat belt is 5m, and the cushion time of seat belt1.2s. What is the average impact force on seat belt? (g= 10m/s2)

(5) Summary

Through the study of this lesson, we understand the characteristics of momentum theorem, and will use it to explain some life phenomena, and we can also use it to solve some mechanical problems such as blows and collisions, and further realize that momentum theorem is closely related to our lives.