"Newton's Third Law of Motion" Newton's Laws of Motion PPT download

"Newton's Third Law of Motion" Newton's Laws of Motion PPT download

The first part of the content: [Learning literacy·clear goals]

Physical concepts: 1. Know that the effects of forces are mutual, and know the concepts of action force and reaction force. 2. Understand the exact meaning of Newton's third law, and be able to use it to solve simple problems.

Scientific thinking: 1. Be able to use Newton’s third law to solve related problems. 2. Be able to distinguish between equilibrium forces, action forces and reaction forces.

Newton's third law of motion PPT, part 2: independent preview and exploration of new knowledge

Knowledge sorting

1. Action force and reaction force

1. Definition: A pair of interaction forces between objects is called ______ and reaction force. Any one of the forces can be called an action force, and the other force is called ______.

2. Characteristics: The effect of force is always ______. Mutual action and reaction forces can exist between objects that ______ each other, or between objects that ______.

2. Newton’s third law

1. Content: The magnitude and direction of the action force and reaction force between two objects are ______, and they act on the same straight line.

2. Expression: F=______, where F and F′ represent action force and reaction force respectively. The negative sign represents two forces______.

Basic self-test

1. Think and judge (mark “√” if correct, “×” if incorrect)

(1) Force is the effect of an object on an object. The object exerting force is also the object receiving force. ( )

(2) The object receiving the action force and reaction force is the same object. ( )

(3) The resultant force of action force and reaction force is zero. ( )

(4) A person is punching hard. It can be seen that a force can only have a force-exerting object but not a force-receiving object. ( )

(5) There is an action force first and then a reaction force. ( )

(6) Action force and reaction force must be of the same nature. ( )

2. As shown in the picture, an adult A and a child B stood on the level ground holding hands to compare their strength. As a result, the adult pulled the child over. The correct statement about the relationship between the forces acting on both sides in this process is ()

A． The force an adult pulls on a child must be stronger than the force a child pulls on an adult.

B． The pulling force between adults and children is a pair of action and reaction forces

C. The resultant force of the adult pulling the child and the child pulling the adult must be zero.

D. Only when the adult pulls the child, the adult's force is stronger than the child's force. During a short stalemate that may occur, the pulling force of the two people is the same.

B[The force of the adult pulling the child and the force of the child pulling the adult form a pair of interacting forces. The relationship between them satisfies Newton's third law, so the two are always equal in size and opposite in direction. The two forces act on different On the human body, the resultant force cannot be found. The child is pulled because the pulling force he receives is greater than the friction force he receives. Therefore option B is correct. ]

3. Regarding the understanding of Newton's third law, which of the following statements is correct ()

A． When the force is generated, a reaction force is generated; when the force disappears, the reaction force slowly disappears.

B． Elasticity and friction have reaction forces, but gravity has no reaction force.

C. The force exerted by object A on object B is elastic force, and the reaction force exerted by object B on object A can be friction force.

D. Action and reaction forces are never balanced under any circumstances

Newton's third law of motion PPT, the third part: cooperative exploration to overcome difficult problems

Action and reaction forces

1. Action and reaction forces

Similarities and Differences between Action Force and Reaction Force

Sitong

Same size: two forces have the same size

Same line: acting on the same straight line

Homosexuality: the nature of the force is the same

Simultaneity: simultaneous emergence, change and disappearance

Three different

Opposite direction: Two forces are in opposite directions

Allogeneic

Act on different objects

It cannot be synthesized and cannot be balanced.

Different effects: different effects

2. The most intuitive difference between action force, reaction force and balance force is that the objects receiving the force are different.

[Example 1] (Multiple choice) As shown in the figure, a horizontal force F presses an object stationary against a vertical wall. The correct one of the following statements is ()

A． The action force F and the support force of the wall on the object are a pair of action force and reaction force.

B． The force F and the pressure of the object on the wall are a pair of balancing forces

C. The gravity of the object and the static friction force of the wall on the object are a pair of balancing forces

D. The pressure of the object on the wall and the support force of the wall on the object are a pair of action and reaction forces

Ideas: First analyze the force on the object, find the forces that balance each other, and then find the object exerting force on each force. The force acting on the object applying force is the reaction force.

CD [The force F and the supporting force of the wall on the object act on the same object. They are equal in magnitude and opposite in direction. They are a pair of balancing forces on a straight line. A is wrong. The force F acts on the object and the pressure of the object on the wall acts on the wall. These two forces cannot become a balanced force, nor are they action force and reaction force. B is wrong. In the vertical direction, the object is subject to the vertical downward gravity and the vertical upward static friction force of the wall on the object. Since the object is at rest, these two forces are a pair of balanced forces, C pair. The pressure of the object on the wall and the support force of the wall on the object are the interaction force between the two objects, a pair of action force and reaction force, D pair. ]

For training

1. Which of the following understandings about action force, reaction force and a pair of balancing forces is correct ()

A． The resultant force of a pair of balancing forces is zero, and their effects cancel each other out. The resultant force of a pair of action forces and reaction forces is also zero, and their effects cancel each other out.

B． Action force and reaction force are produced, changed and disappeared at the same time, and have the same nature. The nature of a pair of balancing forces is not necessarily the same.

C. Action force and reaction force arise, change and disappear at the same time. The same is true for a pair of balancing forces.

D. There is an action force first, and then there is a reaction force. However, a pair of balancing forces act on the same object at the same time.

2. As shown in the figure, the object is at rest. Which of the following statements is correct ()

A． The pulling force of the rope on the object and the pulling force of the object on the rope are a pair of action force and reaction force

B． The gravity of the object and the pulling force of the object on the rope are a pair of balancing forces

C. The gravity of the object and the pulling force of the rope on the object are a pair of action and reaction forces

D. The reaction force of the object's gravity acts on the rope

Newton's third law

1. The formula expression of Newton's third law is: F=-F'. The negative sign means that the two directions are opposite, and the equal sign means that the two are equal in size.

2. The physical significance of Newton's third law: It reveals the rules of interaction between objects, reflects the magnitude and direction characteristics of action and reaction forces, and reveals the connection between interacting objects.

3. Issues that should be paid attention to when applying Newton's third law

(1) The "always" in the law indicates that Newton's third law is true for any object and under any conditions.

(2) Newton's third law states that among action force and reaction force, if one arises or disappears, the other must appear or disappear at the same time, otherwise the "mutual relationship" will be violated.

regular method

The scope of application of Newton's third law

Newton's third law applies not only to the interaction between solids, but also to the interaction between gases and liquids. It is also related to the nature of the force, the mode of action (contact or non-contact), the mass of the object, the state of motion and the reference The choice of system is irrelevant.

Newton's third law of motion PPT, part 4 content: Achieve standards in class and solidify the base

1. (Multiple choices) The following are common examples in life and production. The application of Newton’s third law is ()

A． The boat moves forward by rowing

B． Sailing boats use the power of the wind to sail

C. Propeller planes fly by rotating their propellers

D. Jet plane sprays gas backward to propel the plane forward

2. Our country's diving sport is in a leading position in the world. The figure shows the athlete's take-off action before diving. Which of the following statements is correct ()

A． The force exerted by athletes on the board is greater than the support force of the board on them

B． The force exerted by athletes on the board is less than the support force of the board on them

C. Athletes' support is balanced by gravity

D. The net external force on the athlete must be upward

3. Among the following pairs of forces, the one that interacts is ()

A． The pulling force of the hanging rope on the electric lamp and the gravity of the electric lamp

B． The force of the light pulling on the hanging rope and the force of the hanging rope pulling on the light

C. The force of the hanging rope pulling on the ceiling and the force of the electric light pulling on the hanging rope

D. The force of the suspension rope pulling on the ceiling and the gravity of the light

4. (Multiple choices) When a horse-drawn cart is moving, assume that the force exerted by the horse on the cart is F and the force exerted by the cart on the horse is F′. The correct statement about F and F′ is ()

A． F and F′ are a pair of action and reaction forces

B． When the horse and cart accelerate, F>F′

C. When the horse and cart are decelerating, F

D. No matter what movement the horse and cart make, the magnitudes of F and F′ are always equal.

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