1, Newton's first law.
If the external force acting on an object is zero, the motion speed of the object remains unchanged (law of inertia)
Newton's first law shows that if the external force acting on an object is zero, the motion speed of the object remains unchanged. Velocity is a vector, including the magnitude and direction of motion.
According to this law, it can be concluded that a stationary object will remain stationary until an external force acts on it. A moving object will keep its speed and direction until an external force acts on it.
Newton's second law.
The external force acting on an object is equal to the product of its mass and acceleration.
Newton's second law shows that the external force acting on an object is equal to the product of mass and acceleration. This law is also called "the law of acceleration". Expressed as an equation, F=ma, where f is external force, m is mass and a is acceleration.
The second law can also be expressed by momentum, that is, the external force acting on an object is equal to the rate of change of momentum:
f = DP/dt； Where p is momentum and t is time.
Since momentum is equal to mass times acceleration, if the mass is constant, the law of acceleration can be obtained. If the mass easily changes with time, the system is a variable mass system, and the time-varying quality must be considered. Please refer to the variable mass system for more details.
Newton's third law.
When two objects interact, the forces exerted on each other are equal in magnitude and opposite in direction (acting force and reaction force).
Newton's third law shows that when two objects interact, the forces exerted on each other are equal in magnitude and opposite in direction. According to the third law, force is the interaction between objects, and forces will appear in pairs, one of which is called "acting force" and the other is called "reaction force".
These two forces are equal in magnitude and opposite in direction. Between these two forces, any force can be called an action, and its corresponding force naturally becomes an accompanying reaction.
This pair of action and reaction is called "pairing force". The third law is also called the Law of Action and Reaction.
Extended data:
In the past two hundred years, physicists have completed many experiments and observations to test Newton's laws of motion, which can calculate good approximate results for general conditions. Newton's law, Newton's law of universal gravitation and the mathematical methods of calculus give consistent quantitative explanations to various physical phenomena.
In some cases, Newton's laws of motion are not applicable, so more advanced physical theories are needed. Under the condition of ultra-high speed or very strong gravity field, we need relativity to correct and explain some celestial movements and phenomena, such as black holes. On the atomic scale, we need quantum mechanics to explain physical phenomena such as atomic emission spectrum.
But in modern engineering, Newton's law of motion can accurately explain and calculate the problems encountered by engineers for general application cases, such as the movement of vehicles or planes. So Newton's law of motion is still a compulsory basic part for middle school physics, university engineering and science students.
If we want to consider the influence of special relativity, the second law must be revised. Because when the speed is close to the speed of light, the resultant force acting on the object cannot be accurately expressed as the product of the rest mass and the acceleration. Please refer to the project four-dimensional force for details. The third law does not apply to special relativity, because the relativity of simultaneity cannot be realized in the third law.
For two objects that are not in direct contact with each other, but are separated by a finite distance, the third law assumes that the interaction between the objects is instantaneous.
Assuming that two interacting objects are separated by a certain distance, the forces exerted by the two objects at time t are F(t) and -F(t), respectively, as viewed from the reference system A. However, from another frame of reference B with a relative velocity of v≠0, the action time of these two forces is different, so the third law is not valid and needs to be revised.
Baidu Encyclopedia-Newton's Law of Motion