Newton's first law
Newton's first law: Any object remains stationary or in a uniform linear motion until the action of other objects forces it to change this state.
Newton's first law, also known as the law of inertia, tells us that "motion does not require force to sustain it." If an object is subjected to an external force of 0, then what speed it was before and what speed it is after that.
Why does it have a name called the "Law of Inertia"? Inertia is synonymous with inertia in English, which means lazy. Therefore, the law of inertia means that all objects are "lazy", like lazy pigs, are unwilling to actively change their state of motion.
If I don't move now, then stay still unless you push me hard; if I have a speed now, keep going at that speed unless it stops me.
Friends who like science fiction movies are certainly no strangers to this picture: an astronaut accidentally breaks the rope connecting the spacecraft in outer space, and then everyone can only watch the astronaut drift into the depths of space at a certain speed. Because there was no other external force in space to stop him, he could only obey the law of inertia and "lazily" and drift away at the same speed (although he was so reluctant).
This also shows that without force, astronauts can still move, and force is indeed not the reason for maintaining the motion of objects. So, what exactly does force do? Galileo said in the second half of the sentence that force is the cause of changing the state of motion of an object. That is to say, although the motion itself does not need force to maintain, if you want to change the state of motion, such as astronauts do not want to float into the depths of space, want to return to the spacecraft, this requires an external force to pull a hand.
Well, after knowing that force can change the state of motion of an object, I give you a certain force, how much can your speed change? To calculate this account quantitatively, we need Newton's second law.
Newton's second law
How to calculate?
First, since force can change the velocity of an object, it is first necessary to find a physical quantity to describe the change in the velocity of the object. For example, if the car's current speed is 2m/s, and after 1s it becomes 4m/s, then its speed changes within 1s (4–2)m/s = 2m/s.
Why does the speed of the car change? Of course, the traction of the engine changes the speed of the car, and the pull of the horse instead of the horse changes the speed of the car. Since the speed has changed, there must be some force acting on it.
So, what does the magnitude of the force have to do with changes in velocity? Is it that the greater the force, the greater the change in speed? At first glance, it seems to be no problem. With two horse-drawn carriages, the carriage can be accelerated by 2m/s in 1s; if there are four horse-drawn carriages, the carriage may be accelerated by 4m/s in 1s. However, even if there is only one horse, if there is enough time, such as the 60s, it is entirely possible for the carriage to increase its speed even more. Therefore, the amount of change in the ratio of light to the speed is unfair, and it is necessary to limit the time: the greater the change in the speed of the object in a certain period of time (such as 1s) (that is, the faster the speed changes), the greater the force received.
Therefore, there is reason to believe that the greater the external force, the faster the speed of the object changes. The amount of change in the velocity of an object per unit time is the definition of acceleration. That is, we use the physical quantity of acceleration to describe how fast the speed of an object changes. If the velocity of the object is constant, its acceleration is equal to 0; if the velocity of the object increases from 2m/s to 4m/s in 1s, then its acceleration is 2m/s; if the speed of the object increases from 1m/s to 7m/s in 2s, then its acceleration should be:
Well, now we know: the greater the external force on an object, the faster its velocity changes, the greater the acceleration.
So, are there other factors that affect acceleration? The same horse, a pulling bike, a pulling truck, do you think their acceleration will be the same? Obviously, the acceleration of an object is related not only to the external force, but also to the mass: the greater the mass, the smaller the acceleration obtained under the same external force, and vice versa. With this foreshadowing, Newton's second law is about to come out. Newton's second law: the acceleration of an object a is proportional to the external force F of the object, and is inversely proportional to the mass of the object m, and the formula is F=ma. Thus, we bring out one of the most important formulas in Newtonian mechanics, and the whole of high school physics can be said to be learning the application of F=ma in various situations.
Newton's second law also gives us a fresh perspective on the concept of "quality." Doesn't the law of inertia say "everything is lazy"? Without external forces, one by one is unwilling to change their state of movement. However, although everyone is very "lazy", "lazy" is also divided into three, six, nine and so on. Some objects are slightly lazy, and a light push changes the state of motion; some are extremely lazy, and they can't be lifted with eight large cars.
So, how do you tell if an object is a little lazy or very lazy? A: According to the quality. Because Newton's second law tells us that the greater the mass of an object, the smaller the acceleration produced by the same external force, that is, the slower the state of motion changes. Therefore, mass becomes a physical quantity that measures the difficulty of changing the state of motion of an object. The bigger the mass, the fatter, the more you don't want to move, and it seems really reasonable to think about it.
From this point of view, let's feel Newton's second law F=ma: external forces (F) want to change the state of motion of the object (a), but the greater the mass (m), the lazier the object, the more insistent on self, the more difficult it is to be changed by the external force (F). Therefore, the stronger the heart, the more difficult it is to be changed by external temptations; the greater the quality, the more difficult it is to be changed by the drive of external forces.
Newton's third law
Newton's third law: The force and reaction force of the two interacting objects are equal in magnitude and in opposite directions (Newton's original phrase "every action has an equal reaction" and does not mention "force"). But because we are discussing Newtonian mechanics, the textbook is directly used as a force and reaction force to express, which is convenient for understanding). This is easy to understand, for example, if I push the wall hard, I will feel that the wall also pushes me hard, and the two forces are equal in magnitude and in opposite directions.
Source: What is High School Physics
Author: Long Tail Jun
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Editor: Zhang Runxin
The reproduced content represents the views of the author only
Does not represent the position of the Institute of Physics, Chinese Academy of Sciences
Source: Origin Reading
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