Physics Guide for Junior High School in Key Schools (Revised Edition)

Author: / Country: Mainland China
Publisher:
Publish Date: 1998-07-01
Features:
4. Newton's Laws of Motion (Law of Inertia) · Learning Requirements for Inertia
1. Understand Newton's Laws of Motion.
2. Understand what inertia is, know that all objects possess inertia, and that inertia is unrelated to the motion state of an object. Be able to use the concept of inertia to explain simple physical phenomena (inertia phenomena in daily life and productive labor).
3. Be able to distinguish between inertia and the Law of Inertia.
Key Learning Points: Be able to use the concept of inertia to explain simple inertia phenomena in daily life and productive labor.
Learning Guidance:
1. Newton's Laws of Motion
Based on the research achievements of Galileo and others, Newton summarized a principle: When no external force acts on an object, it always maintains a state of uniform linear motion or rest. This is Newton's Law of Motion.
Galileo believed that no force is needed to maintain uniform motion. He designed a famous ideal experiment to explain this.
As shown in Figure 3—1, let a ball roll down from point A along a smooth inclined plane starting from rest. If the object is only subject to gravity and not affected by other resistances, then when the ball reaches point B, it will have a certain velocity. The ball will roll up another inclined plane BC. If there is no friction, the ball will rise to its original height. If the angle of the second inclined plane is reduced, as shown in inclined plane BD in Figure 3—1, the ball will need to travel a longer distance to reach the original height. And so on. If the angle of the second inclined plane is continuously reduced until it is horizontal (BE), the ball can no longer reach the original height and will continue to move at a constant speed along the horizontal plane. Thus, Galileo concluded that if a moving object is not obstructed, its velocity will remain unchanged.
Galileo's student, Peri, and others further pointed out that if a moving object is not obstructed, its trajectory will be a straight line, meaning the direction of its velocity remains unchanged.
Later, Newton summarized the achievements of Galileo and others, concluding the Law of Motion.
The Law of Motion is an indirect conclusion and cannot be directly verified through experiments. Galileo's experiment was an ideal experiment, but it was based on reliable facts. Through scientific analysis and reasoning, he grasped the essence from the phenomena and revealed the laws of physics. Therefore, this is also an important method for studying physics problems. When solving problems, it must be clear that the uniform linear motion or rest state described by Newton's Law of Motion refers to an ideal state where no external force acts on the object. In reality, objects under no external force do not exist. The uniform linear motion or rest states observed in daily life are merely states where the object is in force equilibrium.
2. Inertia
From Newton's Law of Motion, we can understand that all objects, when not acted upon by external forces, possess the property of maintaining a state of uniform linear motion or rest. This property of an object is called inertia. Therefore, Newton's Law of Motion is also known as the Law of Inertia.
It follows that force is not the cause of producing or maintaining the motion of an object, but rather the cause of changing the motion state of the object.