"In physics, there are two conditions for cutting an object: speed and energy. For example, if you put a stationary knife in contact with an apple, you don't have to interact with each other, and in order to do so, you have to press the knife or pull the knife to make the knife cut the apple, right?" Lin Ping said.

The classmates nodded after listening to Lin Ping's speech, and then Wang Yige, who was sitting in the corner, raised his hand: "Teacher!"

Wang Yige is very happy to study with these older brothers and sisters in the upper grades, and he can also understand, especially mathematics and physics, and his English level has also improved with Lin Ping's teaching, but the other subjects are slightly worse.

Lin Ping saw that he loved learning so much and took care of him, so he let him sit in the classroom and listen to the lectures with Liu Xiangqian and Liu Qingmei, while Wang Yige always liked to sit in the corner and didn't know why.

Lin Ping smiled and said, "Is there any problem? Wang Yige." ”

Wang Yige stood up and said, "Teacher, you don't have to press the knife, you can also make the knife cut the apple by pressing the apple or pulling the apple on the knife. ”

Lin Ping smiled with satisfaction and said, "Yes, very good! Wang Yige is also a good child who loves to think and uses reverse thinking, you have to learn from him." ”

Liu Xiangqian pursed his lips disdainfully: "I thought of it a long time ago, I don't bother to say it." ”

Lin Ping smiled and continued: "The effect of force is mutual, indeed what Wang Yige said is also right, he thinks from another point of view, I am talking about cutting apples with knives in most cases, but what he is thinking about is using apples to 'cut' the knife, the knife cuts the apple with a positive force, the apple 'cuts' the knife, it is the apple that exerts force on the knife, but it is cut by the reaction force of the knife, but the result of both is the same, the apple is cut, and the knife is intact." ”

"Then why is the teacher like this?" Wang Yige asked curiously.

"This involves the problem of density and hardness in physics, and apples are smaller than knives in terms of density and hardness, so it is apples that are injured when the two collide. Lin Ping said with a smile. "We have learned an idiom called 'hitting a stone with an egg', which means taking an egg to touch a stone, which is a metaphor for using weak forces to collide with strong forces, not estimating one's own strength, and bringing about self-destruction. is also the same reason, whether it is an egg hitting a stone or a stone hitting an egg, the end is that the egg will be destroyed. ”

Liu Xiangqian said positively: "I know, teacher, this is called not having a B number in my heart!"

The classmates were amused by Liu Xiangqian's words.

Lin Ping knocked on Liu Xiangqian's head and continued: "So, classmates, when you encounter something in the future, you must first consider whether you have the strength to do it, don't make fearless sacrifices, and when you find that your strength is not strong enough, you must work hard to make yourself stronger." ”

"But, teacher," Liu Qingmei said at this time, "an egg will always be an egg, a stone will always be a stone, an apple will always be an apple, a knife will always be a knife, and an egg will never touch a stone, and an apple will never touch a knife." ”

"Well, common sense is like this, but physically, it's not. Lin Ping said with a smile.

The classmate glared at Lin Ping and asked, "Why?

Lin Ping said with a smile: "Yes, theoretically speaking, as long as you give the egg a sufficient speed, the egg flies fast enough, and when it collides with the stone, it will cause huge damage to the stone, and can even break the stone." For example, if we throw an egg from a high place, the faster the egg falls, the more its kinetic potential energy increases, and finally it can smash a person to death or smash the ground open. The same is true for Apple, which can bend hard blades at enough speed. ”

Lin Ping looked at Liu Qingmei and said, "So, even if you are just an egg or an apple, it's not that you can't compete with stones and knives, it's just that you need to put in more effort and determination." ”

Liu Qingmei nodded.

"So," Lin Ping continued, "we must remember when we can live in high-rise buildings in the future: don't throw objects from high altitudes! This is a very immoral thing, and it will cause great harm to pedestrians." Even a pen falling from a high altitude can be fatal to us. ”

The classmates nodded, taking Lin Ping's words to heart.

Wang Yige asked again: "Then the teacher normally hits the knife with an apple, and the knife will not be hurt at all?"

Lin Ping said with a smile: "Yes, the knife will be damaged, but the damage it takes is almost negligible, but after the accumulation of time, the damage suffered by the knife will be reflected, such as the blade becoming dull and there are gaps." There is an idiom called water dripping through stones, even a drop of weak water, over time can drip through the stone, which is exactly this truth. ”

Lin Ping pointed to the ceiling fan and said: "Let's go back to the question of the ceiling fan, I just said that there are two conditions for object cutting: speed and strength. Just like the egg mentioned above, with enough speed and power, can break through boulders, and the same is true for cutting. ”

Lin Ping picked up a piece of paper from the table: "Look, this is just an ordinary piece of paper, but as long as it rotates fast enough, it can also cut our skin, and it will hurt us as seriously as a knife." And let's look at the fan again, the fan blade of the fan is a thin piece of iron, then it will be more powerful to rotate, you must not be curious to touch the rotating fan, the skin will be cut, the muscle will be injured and the blood will flow, the finger will be broken, and even the fan blade will cut directly into your body, one knife or two will not be, but you will most likely not survive!"

The students looked at the spinning fan overhead with fear and wariness.

"That'...... Will he do the same when he falls, teacher?" Liu Xiangqian was so frightened that he moved from under the fan to the seat next to him.

Lin Ping shook his head: " If the fan falls, it will generally tear off the wire, the moment the wire is broken, there is no energy to let the fan continue to rotate at high speed, and it continues to rotate only by relying on inertia, the energy generated by inertia is not enough to occur like Liu Xiangqian imagined flying around in the air scene, the horizontal inertia force of the falling fan is far less than the gravity force, so it will fall vertically, at this time it will cause gravity blow to the person sitting underneath, and the inertial rotation will also produce cutting injuries to the surrounding people, but because there is no continuous energy supply, this inertia rotation will stop after hitting the human bone。 ”

"What if the wires are constant?" Liu Xiangqian asked again.

"Generally speaking, the falling wire of the ceiling fan must be broken, because the fragile wire simply cannot withstand the energy of the falling fan, assuming that the ceiling fan falls and the wire continues, then the danger is indeed very high, but this situation does not need to be considered, there is no practical significance, unless someone wants to murder you. Lin Ping also explained a little too much, this group of children is really imaginative, and always puts forward all kinds of hypotheses, and if this assumption continues, it will be useless.

Afraid of something, I saw Liu Xiangqian ask again: "Then what if the speed of this fan exceeds the fifth gear, and the wire falls down......"

"Don't ask, if you continue to ask like this, there will be only one word: death. As soon as the people lay down, the cloth was covered, and the whole village waited for the food to be served; as soon as the pit was dug and the soil was buried, relatives and friends began to cry. Lin Ping said.

At this time, Liu Weiguo also mentioned Lin Ping and said: "Liu Xiangqian, why do you have so many questions?

"Why did I raise the bar?" Liu Xiangqian glared at Liu Weiguo unconvinced.

"Okay, anyway, you remember three points: first, high-speed rotating things are very dangerous, not only fans, but also other things, you must stay away, there will be life danger; second, the ceiling fan falling this kind of thing is basically impossible to happen, study hard don't stare at the ceiling fan all day long, but at the same time, if you hear the ceiling fan has abnormal noise or abnormal conditions, you should report to the teacher in time, let the teacher ask someone to check; finally, there is physics everywhere in life, the physics knowledge that the teacher told you today is only superficial, you must study physics well in the future, so that you can be yourselfAnswer all kinds of doubts in life for others. Lin Ping said concludingly.

[System: Students' safety awareness has been greatly improved. 】

[System: Students' interest in physics has increased dramatically. 】

[System: Student 'Wang Yige''s interest in physics has become a passion.] 】