"Scientific Exploration: Synthesis of Forces" Force and Balance PPT (Lesson 1 Synthesis of Forces)

Description

"Scientific Exploration: Synthesis of Forces" Force and Balance PPT (Lesson 1 Synthesis of Forces)

Part One: Learning Objectives

1. Know the concepts of common point force, composition of force, resultant force and component force.

2. Understand the parallelogram rule and know the fundamental difference between vectors and scalars.

3. Understand the relationship between resultant force and component force, and master the method of finding resultant force.

Synthesis of scientific inquiry power PPT, part 2: exploration of new knowledge

1. Synthesis of common point force

1. Common point force: If several forces act on the _________ of an object at the same time or their lines of action intersect at _________, these forces are called common point forces.

2. Resultant force: The _________ produced by the combined action of several common-point forces can be replaced by one force. Then this force is called the _________ of those forces, and those forces are called the components of this force.

3. The synthesis of forces: Finding the _________ of several forces is called the synthesis of forces.

2. Parallelogram Rule

1. Parallelogram rule: Let the directed line segment representing two common point forces at an angle to each other be _________ to construct a parallelogram, then the directed line segment corresponding to the _________ between the two adjacent sides represents these two common points. The resultant force of point forces is _________ and _________.

2. How to synthesize more than two forces: Use the __________ rule to find the resultant force of two of the forces, and then use the parallelogram to find the resultant force of this resultant force and the third force, until all external forces are synthesized, and finally we get The resultant force of these forces.

self-test

Thinking Analysis

(1) Several forces acting on the same object must be co-point forces. ()

(2) Forces acting at the same point on an object are copoint forces. ()

(3) The effect of two common point forces acting together is the same as the effect of their resultant force. ()

(4) Sometimes the parallelogram rule is used for the synthesis of forces, and sometimes the triangle rule is used, but the two are different. ()

(5)The resultant force is greater than each component force. ()

(6) The magnitude of the two component forces is constant, the greater the angle, the greater the resultant force. ()

basic understanding

(1) "Cao Chong calls Xiang" is an allusion that everyone is familiar with. Please think:

On what basis does Cao Chong judge that the gravity of the elephant and the stone on the ship are equal? What thinking methods are involved? Please give another similar example based on your life experience.

(2) Is the resultant force always greater than the force? Two forces F1 and F2 of a certain size, when will their resultant force be the largest? When is it smallest?

Tips: (1) When the waterline of the ship is the same, the gravity of an elephant is equal to the gravity of a pile of rocks. It includes the idea of ​​equivalent substitution, that is, an elephant and a pile of stones have the same effect. For example: a bucket of water can be lifted by an adult alone or by two children together.

(2) Not necessarily. The resultant force may be greater than the component force, or may be smaller than the component force, or the resultant force may be equal to the component force; the maximum resultant force of two forces in the same direction is F1+F2, and the minimum resultant force when two forces are in the opposite direction is |F1-F2|.

Synthesis of scientific inquiry power PPT, the third part: teacher-student interaction

synthesis of force

Problem guide

Consider the resultant force on the car in the following situations. (assuming F1>F2)

(1) Two people pull the car in opposite directions

(2) One person pushes the cart and one person pulls the cart

(3) Two people pull the cart at an angle to each other

[Key Points] (1)F1－F2　(2)F1＋F2

(3)F1－F2≤F≤F1+F2

【Core Deepening】

1. The "Three Natures" of Resultant Force and Partial Force

2. The relationship between resultant force and component force

When the magnitude of the two force components remains unchanged, the resultant force F decreases with the increase of the angle θ between the two force components, and increases with the decrease of θ. (0≤θ≤180°)

(1) When the two component forces are in the same direction (θ = 0), the resultant force is maximum, F = F1 + F2, and the resultant force and the component force are in the same direction.

(2) When the two component forces are in opposite directions (θ=180°), the resultant force is the smallest, F=|F1-F2|, and the direction of the resultant force is the same as the direction of the larger component force.

(3) The value range of the resultant force: |F1-F2|≤F≤F1＋F2.

Key Capability 1: The relationship between resultant force and component force

(Multiple choice) Regarding F1, F2 and their resultant force F, which of the following statements is correct ()

A. The resultant force F must have the same effect as the combined effect of F1 and F2.

B． The two forces F1 and F2 must be of the same nature.

C. The two forces F1 and F2 must be forces on the same object.

D． The two forces F1, F2 and F are the three forces that the object receives at the same time.

Key Ability 2: Scope of Synthesis of Two Forces

(Multiple choices) The magnitudes of the two common point forces are F1=6 N and F2=10 N respectively. Then their resultant force may be ()

A. 3N　B. 5N

C. 10N D. 20N

Key Ability 3: The scope of the synthesis of the three forces

There are three forces with magnitudes 13 N, 3 N, and 29 N respectively. Then the maximum and minimum values ​​of the resultant force of these three forces should be ()

A. 29 N, 3 N B. 45N, 0N

C. 45 N, 13 N D. 29N, 13N

Error-prone question types

Three points to note when analyzing the synthetic range of force

(1) The value range of the resultant force of two forces F1 and F2 at the same point is |F1-F2|≤F≤F1+F2.

(2) When three forces are combined, if the magnitude of the third common point force is within this range, then the resultant force of these three forces can be zero.

(3) When calculating the minimum value, it is easily affected by the fixed thinking. Just like finding the maximum resultant force, the three forces are limited to a straight line and the minimum resultant force of the three common point forces is considered, which leads to errors.

[Exercise to achieve standards]

1. Three forces with sizes 5 N, 7 N and 9 N are combined, and the range of the resultant force F is ()

A. 2 N≤F≤20 N B． 3N≤F≤21N

C. 0≤F≤20 N D． 0≤F≤21N

2. As shown in the figure, the resultant force F of two common point forces changes with the angle θ between the two common point forces. Then the magnitude of the two forces is ()

A. 1 N and 4 N B. 2N and 3N

C. 1 N and 5 N D. 2N and 4N

How to find the resultant force

【Core Deepening】

1. Drawing method

Use a drawing tool to draw a parallelogram according to the parallelogram rule, and then use a measuring tool to measure the magnitude and direction of the resultant force. The specific ideas are as follows:

2. calculation method

(1) When the two components of force are collinear

① If the two forces F1 and F2 are in the same direction, then the resultant force F = F1 + F2 has the same direction as the two forces.

② If the two forces F1 and F2 are in opposite directions, the resultant force F = "F1 - F2", and the direction is the same as the larger of the two forces.

(2) When the two force components are not collinear: You can draw a schematic diagram of the component force and the resultant force according to the parallelogram rule, and then use geometric knowledge to solve the diagonal, which is the resultant force. The following are three special cases of finding the resultant force:

Key Ability 1: Synthesis of Two Forces

As shown in the picture, two people work together to pull a plaque up the wall. One of them used a pulling force of 450 N, and the other person used a pulling force of 600 N. If the angle between the pulling forces used by the two people is 90°, find their resultant force.

[Ideas] The pulling force of a person is the two forces on the plaque. Their magnitude and direction are clearly defined, and the resultant force can be obtained using diagramming or calculation methods.

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Update Time: 2024-10-03

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