by Alexe Marin (1814-1895)
excerpt TALKS ON MECHANICSGRAVITY Gheorghe. I should like to ask you something, uncle!Uncle Patru. What is that, my lad?G. I should like to know why this stone falls from my hand as soon as I stop holding it.Anna (laughing). Don't you know why your stone falls? Because if you stop holding it then without fail it has to fall.U. P. (To Anna). My girl, I don't think Gheorghe will be satisfied with that answer, and anyway it is not as simple as you reckon. (To Gheorghe) If the stone you have stopped holding falls, the reason is that all bodies are attracted by the earth, towards which they move when there is nothing to resist their motion. This attraction is called gravity, and all bodies are subject to it, because all bodies have weight.G. So I could do the same thing as I did with the stone with any other body?U. P. Undoubtedly, you can try.(Then Gheorghe and the other children take different objects and play with them, dropping them from their hands. Ilie climbs onto a chair: in one hand he has a stone and in the other a sheet of paper. He lifts his arms as high as he can and releases the two objects. The stone falls in an instant, while the paper reaches the ground shortly afterwards.)Ilie. Uncle, look at what I'm going to do!(He then begins the experiment. Uncle Patru, Gheorghe and Anna watch him.)Why does the paper take longer, but the stone falls straight away?U. P. That is because of the air.I. How is that?U. P. Children, let us go into the garden, and there we shall do an experiment that will help me to explain to you what Ilie has asked me.(Uncle Patru takes the children to a pond and starts talking to Gheorghe.)U. P. Do you know how deep the water in this pond is?G. But, uncle Patru, we measured it this morning and found out it's two yards deep.U. P. Well then, my lad, take this stone and climb this here stepladder so that your arm is two yards above the ground. Ilie, get a stone and hold it at the surface of the water, just enough to wet it ever so slightly. I will clap my hands three times, and on the third clap, both of you let go of the stones at the same time. Let's see which one gets there quicker, to the ground or to the bottom of the pond. Pay attention: one… two… three.A. Gheorghe's stone is on the ground.I. (A moment later) And mine's reached the bottom.U. P. Let's see then, Gheorghe. Can you tell me why your stone got there quicker than Ilie's?G. Because the water resisted his stone going down.U. P. That's right, my lad. And the air likewise resists falling bodies, the same as water resists, although the air resists with a lesser force.G. I understand, but why didn't the air resist the falling stone just now, the same as it resisted the sheet of paper?U. P. The air resisted both. But because the sheet of paper had a large surface area for its small weight, and, as I say, each point of the paper was in contact with the air, while the stone had a much smaller surface area for its much greater weight, the resistance was much greater in the first case, and the paper had to take longer to fall than the stone.G. I can see that. But there is something in what you have told us that I find confusing.U. P. Let's see what you find confusing.G. You told us that all bodies are heavy, but for all that, there are some that rise instead of falling, such as smoke.A. Ah! That's right, uncle: look at that chimney over there, and how the smoke is billowing up.U. P. I see it, Anna, and I shall tell you the reason why. Gheorghe, take this piece of wood and throw it into the pond as hard as you can… Well then… Tell me what you saw.G. The wood sank at first and then it rose.U. P. Do you know why it rose?G. No, uncle.U. P. Because it is lighter than water and if Ilie's stone went straight to the bottom of the pond, the reason is that stone is heavier than water.G. Now I know why smoke rises.I. Why?G. Because it is lighter than air.U. P. Quite so. 1842
by Alexe Marin (1814-1895)
by Alexe Marin (1814-1895)