"End of Chapter Review Lesson" Newton's Laws of Motion PPT

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"End of Chapter Review Lesson" Newton's Laws of Motion PPT

Part One: Consolidation Layer Knowledge Integration

[Core Quick Fill]

1. The relationship between force and motion: Force can _____ the motion state of an object.

2. Newton's first law: All objects remain at rest or __________ unless something _____ forces it to change this state.

3. Inertia is the __________ of an object, and has nothing to do with the object's motion state and force. The size of the inertia only depends on the _____ of the object.

4. Newton's second law: The magnitude of an object's acceleration is _____ to the magnitude of the combined external force, _____ to the mass of the object, and the direction of acceleration is the same as __________. expression:_____.

5. Newton's third law: The action and reaction forces between two objects always have magnitude _____, direction _____, and act on the same straight line.

6. Two basic problems in dynamics

(1) Determine the motion of the object from the force: If the force of the object is known, the acceleration of the object can be obtained by __________, and then the motion of the object can be determined by __________.

(2) Determine the force from the motion: If the motion of the object is known, the acceleration of the object can be found based on __________, and then the force on the object can be determined based on __________.

7． Overweight and weightlessness

(1) Overweight phenomenon: The pressure of an object on the support (or the pulling force on the suspended object) _____ the gravity exerted by the object, and the direction of acceleration _____.

(2) Weightlessness phenomenon: The pressure of an object on the support (or the pulling force on the suspended object) _____ the gravity exerted by the object, and the direction of acceleration _____.

End-of-chapter review lesson PPT, part 2 content: Strengthening the ability of the promotion layer

Solving Connected Body Problems Using Whole Method and Isolation Method

1. Holistic method: When studying the relationship between acceleration and force of a connected body, the connected body is often regarded as a whole. For Newton's second law, F=ma, F is the total external force on the whole, and ma is the corresponding effect of the whole and the external force. Be careful not to include internal forces in the analysis of the overall force.

2. Isolation method: It is mostly used when solving the interaction force of a connected body, that is, if a certain part is separated from the connected body, the force exerted on it by other parts becomes an external force.

3. When solving connected body problems, the overall method and the isolation method must not be opposed. In most cases, the two methods should be used together. When finding the acceleration or resultant force of a connected body whose parts have the same acceleration, the overall method is preferred. If the force between objects is also required, the isolation method is used. In practical applications, these two methods should be used flexibly and alternately according to the specific situation, and should not be stuck to a fixed model. Regardless of whether the overall method or the isolation method is used, the key to solving the problem is to conduct a correct force analysis of the research object.

[Example 1] (Multiple choice) Blocks A and B with masses of 2 kg and 3 kg respectively are placed on a smooth horizontal surface and connected with light springs. As shown in the figure, blocks A and B are now subjected to opposite directions. The horizontal forces F1 and F2 are, and F1=20 N, F2=10 N, then the following statement is correct ()

A． The elastic force of the spring is 16 N

B． If only F1 acts, the elastic force of the spring becomes 12 N

C. If the spring is replaced by a lightweight rope, the pulling force of the rope on the object will be zero.

D. If F1=10 N and F2=20 N, the elastic force of the spring remains unchanged.

[One word to clear customs]

Types of issues commonly involved in the holistic approach and the isolation approach

(1) Issues involving pulleys: If the tension of the rope is required, the isolation method is generally used.

(2) Connector problems on the horizontal plane: This type of problem is generally that the objects in the connected body (system) remain relatively stationary, that is, they have the same acceleration. When solving problems, the method of overall analysis first and then isolation is generally adopted.

(3) Problems with the connection between the inclined plane and the object: When the object has acceleration along the inclined plane and the inclined plane is stationary relative to the ground, the isolation method is generally used for analysis.

Application of Newton's second law to critical and extreme value problems

In some physical situations, during the change of the object's motion state, due to changes in conditions, there will be a connection between the two states and a boundary between the two phenomena. At the same time, a certain physical quantity will have a maximum or minimum value in a specific state. ． Such problems are called critical and extreme value problems.

The critical extreme value problem is a common problem in dynamics. Commonly used solutions include: (1) Limit method: When words such as "maximum", "minimum" and "just" appear in the question, critical problems are generally implied. To deal with this When solving similar problems, the physical problem (or process) can be pushed to the extreme, so that the critical phenomenon (or state) appears and the purpose of rapid solution is achieved.

(2) Hypothesis method: There are no obvious clues of a critical state in some physical processes, but a critical state may or may not occur during the change process. To answer such questions, the hypothesis method is generally used.

(3) Mathematical method: Convert physical processes into mathematical expressions, and solve critical conditions based on mathematical expressions.

[Example 2] A small ball B with a mass m is fixed at two points A and C of the carriage with two equal-length strings 1 and 2 respectively. As shown in the figure, it is known that when the two strings are straightened, the two strings The angle with the front wall of the carriage is 45°. Find:

(1) When the car makes a uniformly accelerated linear motion to the left with acceleration a1 = 12g, the magnitude of the pulling force of the two ropes 1 and 2;

(2) When the car moves in a straight line with uniform acceleration to the left with acceleration a2 = 2g, the magnitude of the pulling force of the two ropes 1 and 2.

[One word to clear customs]

When solving such problems, you must find the critical point, analyze the force and motion of the object from the critical point, and see which quantities have reached the extreme value, and then apply Newton's second law to the critical state combined with the holistic method and the isolation method. Just find the solution.

Two basic problems in dynamics

1. The force on the object is known, and the motion of the object is studied. That is, when the force on the object is known, the acceleration of the object is found, and the motion of the object is determined by combining the kinematic formulas.

It is known that the force on the object ��→A�� is obtained from F=ma→S, v0, vt, t are obtained from the formula of uniform linear motion

2. When the motion of the object is known, study the force on the object. That is, when the motion of the object is known, the acceleration of the object is obtained by the kinematic formula, and then the force on the object is determined from the acceleration.

It is known that the motion of the object �� → Obtain a�� from the formula of linear motion at a constant speed → Obtain the force of the object from F=ma

[Example 3] As shown in the picture, He Wenna is in the trampoline competition. It is known that He Wenna's weight is 49 kg. Assume that the interaction time with the trampoline after she falls freely from a height of 3.2 m is 1.2 s, and the height she rises after leaving the trampoline is 5 m. Find her average force on the trampoline. (g is taken as 10 m/s2)

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Update Time: 2024-10-02

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