Newton's law of motion is Newton's first law of motion, that is, the law of inertia, which was summarized by Sir isaac newton in the17th century and published in Mathematical Principles of Natural Philosophy. Newton's second law of motion and Newton's third law of motion are three basic laws of classical mechanics.
[Edit this paragraph] Newton's first law of motion
In any case, all objects always remain stationary or move in a straight line at a constant speed when they are not affected by external forces.
[Edit this paragraph] [Content]
In any case, all objects always remain stationary or move in a straight line at a constant speed when they are not affected by external forces.
All objects always keep moving in a straight line at a uniform speed or at rest until an external force forces them to change this state.
This is Newton's first law.
Newton's first law can also be abbreviated as: the mover keeps moving and the static keeps static.
Unless it is acted by an external unbalanced force, the particles will remain stationary or move at a uniform speed.
[Edit this paragraph] [Description]
All objects tend to stay still and move in a straight line at a constant speed, so the motion state of an object is determined by its motion speed. Without external force, its motion state will not change.
The property that an object maintains its original state of motion is called inertia, and the size of inertia is measured by mass.
So Newton's first law is also called the law of inertia.
Newton's first law also clarified the concept of force.
It is clear that force is the interaction between objects, and it is pointed out that force changes the motion state of objects.
Because acceleration describes the change of object's motion state, force is related to acceleration, not to speed.
If we don't pay attention to this point in our daily life, we are often prone to illusion.
[attention]
(1) Newton's first law does not hold true in all reference frames, but actually only in inertial reference frames.
Therefore, Newton's first law is often regarded as a criterion whether a reference system is an inertial reference system.
(2) Newton's first law is obtained by analyzing facts, further generalizing and reasoning.
Everything around us is affected by this force or that force, so it is impossible to verify this law directly through experiments.
But all the inferences from the law have stood the test of practice, so Newton's first law has become one of the basic laws of mechanics recognized by everyone.
Discovery and summary of Newton's first law
More than 300 years ago, Galileo analyzed similar experiments and realized that the smaller the resistance, the slower the speed drop and the longer the moving distance.
He further concluded through further reasoning that in an ideal situation, if the horizontal plane is absolutely smooth and the resistance of an object is zero, its speed will not slow down, but it will always move at a constant speed.
Galileo once studied this problem, and Newton once said, "I succeeded on the shoulders of giants." This sentence is directed at Galileo.
So Newton summed up the previous research results and the famous Newton's first law.
[Edit this paragraph] Newton's second law of motion
[Edit this paragraph] [Content]
The acceleration of an object is directly proportional to the resultant force acting on the object and inversely proportional to the mass of the object, and the direction of acceleration is the same as that of the resultant force.
[Edit this paragraph] [emoticon]
∑F=ma or F=ma.
[Edit this paragraph] [Description]
(1) Newton's second law is the instantaneous law of force.
Force and acceleration occur at the same time, change at the same time, and disappear at the same time.
(2)F=ma is a vector equation, and the positive direction should be specified when it is applied. All forces or accelerations in the same direction as the positive direction are positive, and vice versa. In general, the direction of acceleration is positive.
(3) According to the principle of independent action of force, when Newton's second law is used to deal with the motion of an object in a plane, the force acting on the object can be decomposed orthogonally, and the component forms of Newton's second law can be applied in two mutually perpendicular directions: Fx=max and Fy=max.
[Edit this paragraph] [Five properties of Newton's second law]
(1) Homomorphism: F- union, where M and A correspond to the same object.
(2) Vectorality: both force and acceleration are vectors, and the acceleration direction of an object is determined by the combined direction of external forces on the object.
In the mathematical expression ∑F = ma of Newton's second law, the equal sign not only means that the values on the left and right sides are equal, but also means that the direction of acceleration of an object is the same as that of the external force.
(3) Instantaneity: When the external force acting on an object (with a certain mass) suddenly changes, the magnitude and direction of the acceleration determined by this force also suddenly changes; When the external force is zero, the acceleration is also zero, and the acceleration and the external force are in one-to-one correspondence.
Newton's second law is the law of instantaneous correspondence, which shows the instantaneous effect of force.
(4) Relativity: Nature has a coordinate system. In this coordinate system, when an object is not stressed, it will maintain a uniform linear motion or a static state. This coordinate system is called inertial reference system.
Both the ground and objects moving in a straight line at rest or at a constant speed relative to the ground can be regarded as inertial reference frames, and Newton's law only holds in inertial reference frames.
(5) Independence: Every force acting on an object can independently generate an acceleration, and the sum of the accelerations generated by each force is equal to the acceleration generated by the resultant force.
[Edit this paragraph] [Scope of application]
(1) only applies to objects moving at low speed (slower than the speed of light).
(2) It only applies to macroscopic objects, and Newton's second law does not apply to microscopic atoms.
(3) The frame of reference shall be an inertial system.
[Edit this paragraph] Newton's third law of motion
[Edit this paragraph] [Content]
The acting force and reaction force between two objects are on the same straight line, equal in size and opposite in direction.
[Edit this paragraph] [emoticon]
F=-F '
(f stands for action, f' stands for reaction, and negative sign stands for reaction. F' is opposite to action.)
[Edit this paragraph] [Description]
To change the motion state of an object, other objects must interact with it.
The interaction between objects is embodied by force.
It is pointed out that the role of output is mutual, and there is a reaction when there is a role.
They act on the same straight line, equal in size and opposite in direction.
Newton's third law
Content: The acting force and reaction force between two objects are on the same straight line, equal in magnitude and opposite in direction.
Note: To change the motion state of an object, other objects must interact with it.
The interaction between objects is embodied by force.
It is pointed out that the role of output is mutual, and there is a reaction when there is a role.
They act on the same straight line, equal in size and opposite in direction.
It should also be noted that:
(1) has no primary and secondary effects and reactions.
Produce at the same time and disappear at the same time.
(2) This pair of forces acts on different objects and cannot be offset.
(3) Action and reaction must have the same nature.
(4) Independent of the frame of reference.
Newton's third law:
The action and reaction between two objects are always equal in size and opposite in direction, and act on the same straight line.
F 1=-F2
The (1) forces are interactive.
Appear at the same time and disappear at the same time
② The interaction force must be the same force.
(3) The acting force and reaction force act on two objects, and the resulting effects cannot cancel each other out.
④ Action can also be called reaction, but the selected reference objects are different.
⑤ The acting force and the reaction force cannot be combined, because the acting point is not on the same object.
2. The difference between interaction force and balance force
(1) The interaction force is equal in magnitude and opposite in direction, and acts on two objects on the same straight line; The nature of these two forces is the same.
(2) Balance force is two forces acting on the same object, with the same size and opposite directions, and acting on the same straight line.
The nature of these two forces can be different.
(3) Two balanced forces can exist independently, but the mutual forces exist and disappear at the same time.
For example, if an object is placed on a table, the gravity and supporting force on the object belong to the balance force. After the object is taken away, the supporting force disappears, but gravity still exists.
When the object is on the table, the supporting force of the object and the pressure of the desktop are a pair of acting force and reacting force. After the object was taken away, both of them disappeared.
[Edit this paragraph] [Scope of application]
Newton's law of motion is based on the so-called distance action and the corresponding distance action in absolute space-time, which means that separated objects do not need any medium or time to transmit their interaction.
In other words, the interaction is transmitted at infinite speed.
In addition to the above basic ideas, in Newton's time, people understood the interaction.
For example, gravity, magnetic force between magnets and force between objects in contact are all along the connecting line of interacting objects, and the moving speed of interacting objects is within a constant speed range.
In this case, Newton discovered the third law from the experiment.
"Every action always has an equal reaction and its relative resistance; In other words, the interaction between two objects is always equal, and each object points to the other object. " Action and reaction are equivalent, opposite and * * * line, they interact, produce at the same time, and have the same nature. These are often what we should emphasize when teaching this law.
Moreover, in a certain range, Newton's third law is closely related to the conservation of momentum of material systems.
However, with the development of people's understanding of the interaction between objects, the relationship between electricity and magnetism was discovered in the19th century, and the concepts of electric field and magnetic field were established. Besides the coulomb force interaction between static charges along the connecting line, it is found that the moving charges are also influenced by the magnetic field force, i.e. Lorentz force. The moving charge will excite the magnetic field, so there is an interaction between the two moving charges.
Maxwell (183 1- 1879) completed a comprehensive study of electromagnetic phenomena and its laws from 1855 to 1873, and established a systematic electromagnetic theory. It is found that the electromagnetic action is transmitted through the electromagnetic field at a limited speed (light speed C).
The in-depth development of physics reveals that Newton's third law does not apply to all interactions.
If the Coulomb interaction between electrostatic charges is along the connecting line of two charges, and the electrostatic interaction can be regarded as the action at the distance of "infinite speed", then Newton's third law still applies, then Newton's third law does not apply to the interaction between moving charges.
As shown in the figure, the force that the moving charge B acts on the moving charge A through the excitation magnetic field is (not along the line AB), while the magnetic field of the moving charge A has no force on the charge B at this time (the coulomb force between them is not shown in the figure).
It can be seen that there is no reaction to the action at this time, and the law of action and reaction is invalid here.
Experiments show that the short-range effect of electromagnetic field transmission always has time delay.
Newton's third law obviously does not apply to the interaction with delay effect.
In fact, Newton's third law is effective only when the force of two objects along a straight line (called centripetal force) can be ignored (that is, it can be regarded as a direct action at a long distance).
But in Newton's mechanical system, the law of conservation of momentum, which is closely related to the third law, is a universal natural law.
In the case of electromagnetic interaction, the concept of momentum should be extended from the momentum of physical objects to the momentum containing fields; From the conservation of mechanical momentum of physical particles to the conservation of total momentum of all particles and fields, the law of momentum conservation has become a universal law of conservation.
[Edit this paragraph] [Significance of Newton's Law of Motion]
Newton's three laws of motion form the basis of physics and engineering.
Just as Euclid's basic theorem laid the foundation for modern geometry, Newton's three laws of motion provided the basic theorem for the establishment of physical science.
Newton became the most outstanding scientific giant in the past 1000 years because of the three laws, the discovery of gravity and the creation of calculus.
[Edit this paragraph] [Creation process of Newton's law of motion]
1609, Johannes Kepler discovered that the planets revolve around the sun in elliptical (not circular) orbits.
Since then, scientists have enthusiastically tried to explain these orbits mathematically.
Both robert hooke and John Harley tried, but their mathematical methods failed.
1642 isaac newton was born in Lincolnshire, 60 miles from Cambridge, England.
Isaac is a difficult child.
His father died three months before he was born. He didn't like his stepfather, so he was sent to his grandparents to be raised by them.
However, Newton didn't like anyone-he didn't like his mother, he didn't like his grandparents, and he didn't even like his half-brother.
He often threatened to beat these relatives and burn down the house.
At school, he often violates discipline and gives teachers a headache.
Only one person-William Ayscough noticed Newton's cleverness and potential. He arranged for Newton to study at Trinity College (affiliated to Cambridge University).
Because he was too poor to pay the expensive tuition, Newton worked as a servant for other students and earned money to pay for room and board.
He is always alone and secretive, and others say that he always keeps a straight face and likes to argue with others.
1665 A plague broke out in London, and Cambridge University was forced to close, so Newton returned to his sister's manor in the country.
The manor is very closed, and at the same time it lacks the necessary mathematical tools to describe the ever-changing forces and movements-which are exactly what he is interested in, so he feels very depressed.
He is determined to find out the force that makes an object move (or stand still).
In addition to reading Kepler and Harley's relatively new monographs, Newton also studied the works of Galileo and Aristotle.
He collected the research results and theories of early Greek scholars, which were scattered and often contradictory.
He carefully screened these materials and refined them to find out the universal truths and fallacies.
Newton was very good at screening out a few ideas containing truth from a large number of ideas, and his talent was amazing.
Newton was not an experimenter. He likes to think in his head and do experiments like Einstein.
He will concentrate on one thing for a long time until he gets the answer he needs.
In his own words, he would "put the question in front of him, and then start waiting until the first light appeared, and then gradually became clear, and finally suddenly became clear."
Soon, a question began to bother Newton: What forces caused the movement? He focused on Galileo's law of free fall and Kepler's law of planetary motion.
He was so obsessed that he forgot to eat and sleep that his body was almost on the verge of collapse.
At the beginning of 1666, Newton founded three laws of motion, which created the necessary conditions for him to invent calculus and discover the gravity of the earth.
But it was not until 20 years later that Harley encouraged Newton to write the mathematical principles of natural philosophy that Newton published the three laws he founded.
1684, Jean Picard accurately calculated the size and mass of the earth for the first time.
With these necessary figures, Newton can prove that the real orbit of planetary motion can be correctly calculated by using the three laws of motion and his gravitational equation.
Even with conclusive mathematical evidence, Newton only published "Natural Philosophy and Mathematical Principles" in 1687 at Harley's request and persuasion. The main reason for publishing this book is that robert hooke claimed (falsely claimed) that he had discovered the universal law of motion himself.
Principles of Natural Philosophy and Mathematics has become a highly respected and frequently used publication in the history of science.