"Optimization Summary of this Chapter" Newton's Laws of Motion PPT

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"Optimization Summary of this Chapter" Newton's Laws of Motion PPT

Integration and improvement

Dynamic Connector Problem

1. Connected body: A whole composed of two or more interacting objects with the same acceleration is called a connected body. For example, several objects are stacked together, or squeezed together side by side, or connected together with ropes, thin rods, etc. The commonly used methods when solving connected body problems include the overall method and the isolation method.

2. Holistic method: Treat the entire connected body system as a research object, analyze the external forces on the whole, and use Newton's second law to solve the equations. Its advantage is that it does not involve the interaction forces between objects in the system.

3. Isolation method: Isolate a certain object (or part) in the system as a separate research object, conduct force analysis, and solve equations. Its advantage is that it converts the internal force of the interaction between objects in the system into the external force on the research object, and it is easy to see the force situation of a single object (or part) or the movement of a single process.

4. The selection of holistic method and isolation method

When solving a connected body problem in which all parts have the same acceleration, the overall method should be given priority; if the forces between objects still need to be found, the isolation method should be used. When solving connector problems, as the research object changes, the two methods are often used cross-wise. The general idea is to first use one method to find the acceleration, and then use another method to find the force between objects or the resultant force on the system. Regardless of whether the overall method or the isolation method is used, the key to solving the problem lies in the correct stress analysis of the research object.

Summary and improvement

When the acceleration of all parts of an object is the same and internal forces are not involved, it is relatively simple to use the whole method; if interaction forces between objects are involved, the isolation method must be used. The holistic method and the isolation method often need to be organically combined in more complex problems, which will be faster and more effective.

Dynamic multi-process problems

1. Use the "procedural method" to analyze dynamics problems: a problem-solving method that analyzes the physical processes or different states given in the question in chronological order.

2. The idea of ​​​​solving problems with procedural methods

(1) Divide how many different processes or states there are in the question.

(2) Conduct detailed analysis of each process or state to obtain correct results.

(3) The junction point of the two processes is the key to solving the problem.

3. Advantages: Turn complexity into simplicity, turn diversity into unity.

(2019•Weifang First Intermediate End) As shown in the figure, there is an object with mass m = 1.0 kg at the bottom of a sufficiently long fixed incline with an inclination angle of θ = 37°. Now use a light string to pull the object upward from rest along the incline. , the pulling force F = 10.0 N, the direction is parallel to the inclined surface upward, the rope suddenly breaks after time t = 4.0 s, the kinetic friction factor between the object and the inclined surface μ = 0.25, sin 37° = 0.6, cos 37° = 0.8, g = 10 m /s2.Request:

(1) The speed of the object when the rope is broken;

(2) The movement time from when the rope breaks to when the object returns to the bottom of the slope.

critical problems in dynamics

1. Critical problem: refers to the transition state where a certain physical phenomenon (or physical state) is about to occur or not occur. Words such as "maximum", "minimum", "just enough", "just enough" and "at least" that appear in dynamics problems generally imply the emergence of critical states and imply corresponding critical conditions.

2. Common types of critical problems and critical conditions

(1) Critical conditions for contact and disengagement: The critical condition for contact (or disengagement) between two objects is that the elastic force is zero.

(2) The critical condition for relative stillness or relative sliding: static friction reaches the maximum static friction.

(3) Critical conditions for the rope to break and relax: The tension that the rope can withstand is limited. The critical condition for the rope to break and continue is that the actual tension is equal to the maximum tension it can withstand. The critical condition for the rope to relax is the tension on the rope. is zero.

(4) Critical conditions for maximum acceleration and maximum speed: When an object moves under the action of changing external forces, its acceleration and speed will continue to change. When the resultant force is the largest, it has the maximum acceleration; when the resultant force is the minimum, it has Minimum acceleration. When the acceleration is zero, the object is in a critical state and the corresponding speed reaches the maximum or minimum value.

3. The key to solving the problem: correctly analyze the movement of the object, and judge and analyze the critical state. The unique physical relationship that exists when it is in the critical state is the critical condition.

As shown in the figure, a uniform small ball with a mass m = 1.0 kg is fixed in a rectangular box with two thin wires. Line a forms an angle of 53° with the horizontal direction, and line b is horizontal. The maximum pulling force that the two thin wires can bear is Fm=15 N. (cos 53°=0.6, sin 53°=0.8, g is 10 m/s2) Find:

(1) When the system accelerates uniformly upward in the vertical direction, the maximum value of acceleration is required to ensure that the thin wire is not broken.

(2) When the system accelerates uniformly to the right in the horizontal direction, the maximum value of acceleration is required to ensure that the thin wire is not broken.

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