Newton's first law, that is, the law of inertia, puts forward "keeping a person at rest or moving in a straight line at a constant speed", which is the inherent property of the object itself, that is, inertia. In other words, motion does not need force to maintain, and force is the reason to change the motion state of an object.

So, how to further understand the relationship between motion and force? To sum up briefly: 1. If the resultant force acting on an object is zero, the object will remain stationary or move in a straight line at a uniform speed. At this time, we call it 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. To be sure, if an object is moving at a variable speed, the resultant force of external forces it receives must not be zero.

Students often have several questions that they can't figure out. For example, "an object will remain at a constant speed or at rest without external force or zero resultant force." That's understandable. How can a stationary object move? " Originally, it is static. If it is not subjected to external force, it will continue to be static. Only when it is subjected to a certain force will it change from static to motion. Now in the process of our research, it is no longer stressed, but keeps this speed and advances in a straight line at a constant speed.

For example, the space rocket ejects flame backwards, and the speed is increasing due to the reaction force. This is a process in which the motion state changes from static to dynamic, and the speed changes from small to large. At the last moment, when the powder gas is exhausted and the speed reaches 7.9 km/s, it is no longer stressed; The speed will not change again, and it will be maintained forever, and we will keep moving forward. Of course, in fact, at this speed and height, it is still influenced by the gravity of the earth. The direction of this force is perpendicular to its direction of motion, which makes its direction of motion constantly change. Similarly, because the force is perpendicular to the direction of motion, it will not affect its speed of motion. So generally speaking, it will do uniform circular motion.

One more question. How does a moving object change its state of motion when it is subjected to a non-zero force? It's a long story It can be discussed in several situations. 1. If the force is perpendicular to the direction of motion, such as the rocket and satellite mentioned just now, the satellite will keep its original speed and make a uniform circular motion. 2. If the force is consistent with the direction of motion, then the motion will be faster and faster, that is, to accelerate the motion. For example, after the car starts, the driver continues to increase the throttle, and the traction is consistent with the direction of movement, which will accelerate. On the other hand, when the driver steps on the brake, the car still needs to rush forward for a certain distance. At this time, there is no traction, only friction, that is, the direction of force is opposite to the direction of motion, and then the object slows down. Why is the direction of force backward and the car still moving forward? Because the car has inertia, it should keep its original state of motion, but only because of the force, its state of motion is changed, its motion slows down and finally stops. It can be seen that force is not the cause of object motion, but only the cause of changing the state of object motion. Some students can't figure it out; If you don't play football, it won't move. Is it not forced to move? This statement is correct, but it shows that two points (1) make the ball move; (2) Force changes the motion state of the ball. However, if a flying ball is moving from the beginning within the time and distance we observe it, it can't be said that it only moves under the condition of force, but stops slowly due to the resistance of the ground and air, which still shows that force is the reason for changing the state of motion of the object. Students misunderstand because things around us are actually relatively static. In fact, many objects in the universe are in motion, and motion and stillness are relative. Also, our conclusions and principles are relative to the study of a certain interval. Therefore, the judgment that force is the reason for changing the motion state of an object has reached the essence of the problem.

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