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J. Perelman
"Entertaining physics". Book 2.
Chapter 1. The basic laws of mechanics

DIFFICULT LAW

None of the three basic laws of mechanics is not, perhaps, so much bewilderment, as the famous "Newton's third law" - the law of action and reaction. All know him, know how, even in other cases correctly apply, and but few are free from some of the ambiguities in his understanding. Maybe, reader, you happen to understand it, but I admit it, I grasped it only a dozen years after the first with him Dating.

Talking to different persons, I have found that most are willing to admit the correctness of this law only with considerable reservations. Willing to admit that he is faithful to phone fixed, but don't understand how you can apply it to the interaction of bodies moving... Action, says the law, is always equal and opposite reaction. This means that, if the horse pulls the cart, and the cart pulls the horse back with such force. But then the cart must remain in place: why does it move? Why these forces do not balance one another, if they are equal?

These are the usual confusion associated with this law. Then the law is wrong? No, he's absolutely correct; we just misunderstood him. Forces do not cancel each other simply because attached to different bodies: one to the cart, the other horses. The forces are equal, Yes, but is it the same forces always produce the same actions? Is equal forces will be given to all bodies of equal acceleration? Is the force on the body is independent of the body, the amount of "resistance"that the body itself has the power?

If you think about it, it will become clear why the horse drags the cart while the cart pulls it back with the same force. The force acting on the cart, and the force acting on a horse, in every moment equal; but as the carriage moves freely on the wheels and the horse hits the ground, it is understandable why the cart is rolling in the direction of the horse. Think about that if the cart had no counter force of a horse, then... could be dispensed with and without horses: the weak force would have to bring the cart in motion. The horse is then necessary to overcome the opposition of the cart.

All of this was acquired would be better and would create less confusion, if the law were expressed not in a conventional short form: "action is equal to counteraction", and, for example, so: "opposing force equal to the force acting". After all equal here only force - actions (if you understand how usually understand by "force" moving body) is usually different, because the forces applied to different bodies.

Similarly, when the polar ice squeezing the body of "Chelyuskin", his side pressed on the ice with equal power. The disaster occurred because of a powerful ice was able to withstand this pressure without breaking; the hull of the ship, though, and steel, but not representing a solid body, has succumbed to this force was wrinkled and crushed. (Read more about the physical causes of the death of "Chelyuskin" discussed further in a separate article).

Even the fall of bodies is strictly obeys the law of resistance. The Apple falls to the Ground because it attracts the globe; but with exactly the same force and Apple attracts the whole planet. Strictly speaking, the Apple and the Earth fall on each other, but the rate of decline is different for Apple and for the Earth. Equal forces of mutual attraction tell the Apple acceleration of 10 m/sec2, and the globe - in the same time less the number of times the Earth's mass exceeds the mass of the Apple. Of course, the weight of the globe in an incredible number of times greater than the mass of the Apple, and because the Earth receives moving so insignificant that it can almost be considered equal to zero. That is why we say that an Apple falls to the Ground, instead of saying: "the Apple and the Earth falling on each other".

Entertaining physics J. Perelman

 




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