I. Inertia and Inertia Law

1. Newton's first law (that is, inertia determines Tianjin) (1) Content: When no external force acts on all objects, they always remain at rest or move in a straight line at a constant speed. This is the so-called Newton's first law, that is, if an object is not stressed, the original moving object will maintain its original speed, and the original stationary object will never remain at rest, that is, the object is not stressed. (2) Newton's first law is a law based on Galileo's experiments. It is not an experimental law.

2. Inertia (1) Definition of inertia: All objects have the property of maintaining uniform linear motion or static state unchanged. We call this characteristic inertia. (2) Inertia is an attribute of an object. Any object has inertia under any circumstances.

Two, two force balance

1. What is the balance between two forces (or several forces)?

When an object is acted by two forces (or several forces), we say that the two forces (or several forces) are in balance with each other if it remains in a state of static or uniform linear motion.

2. Conditions for the balance of two forces

(1) condition: If two forces acting on an object are equal in magnitude and opposite in direction, and on the same straight line, then the two forces are balanced. (Extensible to multi-force balance)

(2) Application: ① An object must be in equilibrium under the action of a balancing force. ② When an object is stationary or moving in a straight line at a constant speed, it is bound to be acted by a balancing force. If one force is known according to the equilibrium conditions of the two forces, the magnitude and direction of the other force can be known.

Third, friction.

1. Static Friction (1) When two objects are in contact with each other and are about to move relatively, a force is generated on the contact surface to prevent the objects from moving relatively. This force is called static friction. (2) Before the relative motion of objects occurs, the magnitude of static friction is equal to the magnitude of thrust.

2. Sliding Friction (1) When two objects in contact with each other move relatively, the force that hinders the relative movement of the objects on the contact surface is called sliding friction. (2) Sliding friction is related to pressure and roughness of contact surface. The greater the pressure, the rougher the contact surface and the greater the friction. (3) The direction of sliding friction is opposite to the direction of relative motion of objects. (4) The way to increase beneficial friction is to increase the pressure and roughness of the contact surface. The way to reduce harmful friction is to reduce the roughness of contact surface. For example, lubricating oil is added or the friction surfaces are not in contact; Use rolling instead of sliding.

Fourth, the relationship between movement and force.

1. The change of motion state of an object refers to the change of speed and direction of the object, both of which are called the change of motion state.

According to the law of inertia, an object can keep moving in a straight line at a uniform speed when it is not acted by external force. It shows that the motion of an object does not need force, and force is not the reason for the motion of the object. The change of the motion state of an object is bound to be strongly influenced, so force is the reason for changing the motion of an object.

3. The specific relationship between motion and force is: (1) If the resultant force acting on an object is zero, then the object will remain stationary or move in a straight line at a uniform speed. That is, in a state of equilibrium. On the other hand, if an object is in equilibrium, the resultant force of the external forces it receives must be zero. (2) If the resultant force acting on the object is not zero, then the object will move at variable speed, that is, the magnitude or direction of the object's speed will change. On the other hand, if an object moves at variable speed, the resultant force it receives must not be zero.

The core knowledge of the chapter on force and motion in the part of knowledge mechanics. Inertia, inertia law and two-force balance anti-friction are the key knowledge to be investigated, and they are also the important foundation of physics learning in the future, so we must pay enough attention to them.