Weight of a body when falling

2000/01/01 Bandres Unanue, Luis Iturria: Elhuyar aldizkaria

• Fisika

At first, when the elevator floor begins to drop, and we by stopping it, we have not yet reached its speed, our body barely exerts pressure on the ground, so the feeling of our weight is less than normal. But once pulled out, this special sensation disappears and our body wants to go faster than the elevator (which comes down with a uniform movement), so it presses on the ground, recovering the feeling of its usual weight.

Let's hang some of the key of a dynamometer and when everything descends at once, let's look where the needle is diverted. During this movement we will observe that the weight representing the needle is less than the weight of the needle. And if the dynamometer fell completely free (and we had the possibility to see the result of the needle), we would see that when the needle fell marks zero, that is, as if the pisito had no weight.

We can say that weight bodies lose weight when they fall. But how? To do this, we must first clarify what we understand by weight. The weight of a body in the usual language is the pressure force that exerts on the surface that supports it or, if it is hung, that pulls the key. But the body, when falling, does not pull the key of the dynamometer or therefore its spring, since all have begun to fall. Therefore, the needle will only mark zero.

Galileo, somehow creator of mechanics, XVII. In the twentieth century, his "iscorsi e dimostrazioni matematiche in torno a due nuove scienze attenenti alla meccanica e i movimenti locali" wrote: "We feel a burden on our shoulder when we try to avoid the fall of that burden. But if we start to go down with the same speed as the load on our shoulder, how is it possible that it makes us pressure or discomfort? That and with the same speed as us, one who runs, without letting go of the hand, is like wanting to hurt with a spear."

All this can be verified by a simple experiment. In a saucer of a balance of arms we place a cascante of nuts as follows: while one arm of the nutcracker is on the saucer, the other will be tied with a thread by the key of the balance.

Once this is done, to achieve balance balance, the necessary weights will be placed on the other cymbal. If then we approach the thread a poxpolo on, when the thread burns, the arm of the rattlesnake falls on the saucer.

What will happen on the balance? As long as the arm of the rattlesnake falls, rises, falls or remains in balance? If we have well understood all of the above, we will be able to give a correct answer to this question: this dish will rise.

Of course, the arm of the rattlesnake, when it falls, even if it is on the cymbal, will make less pressure than when it was tied, so the cymbal will rise in the hollow of the fall.

Accurate weighing

The accuracy of the scale we use is often unknown. But what is more important to weigh the balance or weights well? If someone thinks that both are of the same importance, he is wrong. Although the balance is not very precise, a good weighing can be done, as long as we have a high weight. There are several procedures for this.

One of them was invented by the prestigious Russian chemist Mendeliev. This procedure involves using a balance of arms. To begin with, on a balance plate, we have to put something heavier than we have to weigh and balance the weight of that thing with weights on the other cymbal. Once this is done, we will place the body that we want to weigh in this second dish and we will rebalance eliminating the necessary small weights. Of course, the weight of the body we wanted to weigh will be the same as the weight of the body we removed, whether the balance is good or bad.

This procedure is called a "constant weight procedure" and is very comfortable if we have to weigh several bodies. In this case you do not have to change the body initially placed, as it serves for all heavy.

If instead of using the balance of arms, we use the spring balance or dynamometer, once the body is hung despite the key of the dynamometer, we will read the value that marks the needle. Then the body is removed and a few pisitas are placed to the previously marked value. The value of the small weights then placed will give us the weight of the body we wanted to measure.