synonymNewton's first law(Newton's first law) generally refers to Newton's first law of motion (physical theorem)
NewtonThe first law of motion is called Newton's first law for short.also calledinertiaLaws.commonComplete description: any object shall keep moving or stationary at a constant speed in a straight line until it is forced by external forces to change its motion state.[1-2]
Newton's first law and Newton's second and third laws constitute a complete system of Newtonian mechanics.
Newton's first law gives the concept of inertial system, while the second and third laws and the particle mechanics system established by Newton's law of motion are only valid for inertial system.Therefore, Newton's first law is indispensable and a completely independent and important law of mechanics.[4]
Chinese name
Newton's first law of motion
Foreign name
Newton's First law of Motion
Alias
Law of inertia
expression
∑Fi=0→dv/dt=0
Presenter
Isaac Newton
Proposed time
1687
Applicable fields
Relation between force and motion state, elementary physics
Newton in《Mathematical Principles of Natural Philosophy》InOriginal statementYes: Keep everythingUniform linear motionorQuiescent stateUntil an external force forces it to change its motion.[3]This expression has been quoted in senior high school physics textbooks published by People's Education Press and Guangdong Education Press.
It is expressed as:
, where
Is the resultant force,vIs the speed,tIs the time.
The expression in the physics textbook for senior high school published by Shandong Education Press is: Newton's first law shows that when the combined external force is zero, the original static object will continue to remain stationary, and the original moving object will continue to move at the original speed in a uniform straight line.There are two situations when the resultant external force is zero: one is that all external forces on the object cancel each other, and the resultant external force is zero;The other is that objects are not affected by external forces.[5]Some experts and scholars think that this expression is not rigorous, so they usually use the original expression.[2]
Definition of junior high school physics (People's Education Press) in 2019: When all objects are not affected by external forces, they always remain stationary or move at a constant speed in a straight line.
Deductive process
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Galileo's method of studying kinematics is to combine experiments with mathematics, focusing on logical reasoning and relying on experimental verification.[6]His inference of smooth slope is obtained through experimental observation and inference.But this completely smooth slope does not exist in reality, because the friction cannot be completely eliminated, so the ideal slope experiment belongs to Galileo's logical reasoning part.
Galileo's Inference on Smooth Slope
In reality, when a ball rolls downward along an oblique face, its speed increases, while when it rolls upward, its speed decreases.
From this, Galileo deduced that when the ball rolls along the horizontal plane, its speed should not increase or decrease.In fact, he found that the ball became slower and slower, and finally stopped.Galileo believed that this was not its "natural nature", but because of friction, because he also observed that the smoother the surface, the farther the ball would roll.
Galileo's ideal inclined plane experiment
So he reasoned that without friction, the ball would roll forever.
Galileo's ideal inclined plane experiment
Galileo's ideal inclined plane experiment is shown in the figure. Let the ball roll down from a static state along a smooth inclined plane. The ball will roll up another inclined plane, reach the same height as before, and then roll down.He reasoned that it was only because of friction that the ball failed to reach its original height.Then, he reduced the inclination of the latter slope. The ball still reached the same height on this slope, but then it would roll farther.Continue to reduce the inclination of the second slope, and the ball will roll farther when it reaches the same height.
So he studied the situation when the inclined plane was laid flat, and the conclusion was obviously that the ball would roll forever.That is to say, force is not the reason for maintaining the motion of the object, that is, maintaining the speed of the object, but precisely the reason for changing the motion state of the object, that is, changing the speed of the object.Therefore, once an object has a certain speed, if it is not forced, it will move at this speed in a uniform straight line.[7]
Scope of application
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Newton's first law only applies toInertial reference frame。When the particle is not affected by external force, it can be judged that the particle is stationary or moving in a uniform straight lineReference systemIt must be an inertial reference system, so Newton's first law of motion is applicable only in the inertial reference system.
Newton's first law is not applicable in a non inertial reference system (that is, a system with acceleration), because objects that are not subject to external forces may also have acceleration in this reference system, which is contrary to Newton's first law.[8-9]
When Newton's first law does not hold, that isNon inertial frameThe mechanical equation in the non inertial system should be used
Solve mechanical problems.Where
Is the resultant force on the object measured in the inertial system,
Is the inertial force measured in a non inertial system,
Is the acceleration of non inertial system.[4]
Independence
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Newton's first law is a completely independent basic law. Its independence is shown in:
one
DeterminedInertial reference frameAnd it leads to the logical cycle argument, which is the manifestation of the axiom system, and the first proposition of any discipline must have this characteristic.
two
It is pointed out that any object has inertiaInertial concept。[10]
three
Its negative proposition reveals thatConcept of forceThe force is the action of the object on the object, and the force makes the movement state of the object change.
four
yesNewton's second lawFoundation ofFirst, Newton's first law isSecond lawConcept prepared（power、Inertial mass、Inertial frame）The relationship between force and motion is qualitatively clarified;Secondly, the first law mainly explains the state of motion of objects when they are not subject to external forces.It is not the same thing that the object is not affected by external force and the sum of external force vectors is zero. Therefore, Newton's first law cannot be regarded as a special case of Newton's second law when F=0.To sum up, Newton's first law is a completely independent basic law. No other law can solve the problem with it. The second and third laws cannot replace the first law at all.[11]
Development History
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5th century BCDemocritus、EpicurusIt is believed that "when atoms are carried forward in the void and nothing collides with them, they must move at the same speed." This is just the result of speculation or speculation.
In the 4th century BC, Greek philosophersAristotleIt is pointed out that force is the reason for maintaining the movement of objects. If there is force, there will be movement. If there is no force, there will be no movement.Although this is a wrong point of view, he first proposed the argument that there is a relationship between force and motion, which is his contribution to dynamics.[6]
Philopenos, a Greek scholar in the 6th century, was critical of Aristotle's theory of movement.He believed that the projectile itself had some kind of power, which pushed the object forward until it was exhausted. This view later developed into the "impact theory".
In the 14th centuryBridanAlbert, Orisim and others put forward the "Impulse Theory". They believed that: "When the driver pushes an object to move, he will apply a certain impulse or power to it. The higher the speed, the greater the impulse. When the impulse is exhausted, the object will stop." This opened the way for Galileo and Newton.[6]
In the 17th century, Galileo put forward a statement similar to the principle of inertia many times in his own works.In 1632 and 1638, he recorded his ideal slant experiment in the Dialogue on the Two World Systems of Ptolemy and Copernicus and the Talk on the Two New Sciences of Mechanics and Motion (referred to as Two New Sciences for short),And the conclusion is drawn: "If the object falls down along a smooth slope and moves up along another slope, the object will still reach the same height as the original without being affected by the slope angle, but the time required is different."[6]Galileo's conclusion broke the old idea that objects in motion under force would be static when the external force stopped acting for more than a thousand years since Aristotle.Galileo's thought is undoubtedly a big step ahead of his predecessors, which is very closeLaw of inertiaHowever, Galileo has not yet got rid of Aristotle's influence. It cannot be said that Galileo discovered the law of inertia.[6]
In 1644,Descartes In his book Principles of Philosophy, he made up for Galileo's shortcomings.[12]He clearly pointed out that unless an object is affected by external factors, it will always remain stationary or in motion, and specifically stated that an inertia moving object will never make itself move towards a curve, but only keep moving on a straight line.He expressed this basic principle as two laws: first, each individual substanceparticulateIt will continue to maintain the same state until it is forced to change this state when it collides with other particles;2、 All the motions themselves follow a straight line.However, Descartes did not establish the system that he tried to establish to deduce various natural phenomena, many of which were wrong, but his thoughts had a certain impact on Newton's synthesis.Descartes' contribution lies in that he was the first to realize that force is the reason for changing the state of motion of objects.[2]
In 1687,NewtonOn the basis of the work of Descartes, Galileo and others, he wrote the Mathematical Principles of Natural Philosophy, broke away from the shackles of old ideas, and formally put forward the law of inertia as the first principle: all objects always maintain a uniform linear motion or static state, unless the external force acting on it forces it to change this state.He put forward the view that maintaining the state of uniform linear motion and the state of rest are the inherent properties of objects, and the concept of inertial reference frame derived from them.[13]
Law influence
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oneNewton's first law gives a reference system without acceleration——Inertial frame, making people meaningful in studying physical problems and measuring physical quantities,[4]So that it becomes the starting point of the whole mechanics and even physics.[14]Newton's second and third laws and the particle mechanics system established by Newton's law of motion, such asMomentum theorem、Law of conservation of momentum、Kinetic energy theoremAnd so on, only for the inertial system.
twoNewton's first law is the premise and foundation of other principles.The basic concepts contained in the first law lay the conceptual foundation of classical mechanics, thus making it the premise of the first principle in the theoretical system, which is shown in:
(1) For the first time, it refuted theAristotlePeople's wrong concept of force lays the foundation for establishing a correct concept of force.
(2) For the first time, the qualitative definition of force (including the essence and effect of force) is scientifically given.
(3) It is the first time to put forward several basic concepts of classical mechanics, which are established by the second and third laws and Newton's law of motionParticle mechanicsThe system principle laid the conceptual foundation.[15]
