"Galileo formulated it thus: A body remains in the state which it is in, at rest or in motion, as long as no external force compels it to change its state."
"If you say so."
"But this was a significant observation. Since antiquity, one of the central arguments against the earth moving round its own axis was that the earth would then move so quickly that a stone hurled straight into the air would fall yards away from the spot it was hurled from."
"So why doesn't it?"
"If you're sitting in a train and you drop an apple, it doesn't fall backward because the train is moving. It falls straight down. That is because of the law of inertia. The apple retains exactly the same speed it had before you dropped it."
"I think I understand."
"Now in Galileo's time there were no trains. But if you roll a ball along the ground--and suddenly let go..."
"... it goes on rolling ..."
"... because it retains its speed after you let go."
"But it will stop eventually, if the room is long enough."
"That's because other forces slow it down. First, the floor, especially if it is a rough wooden floor. Then the force of gravity will sooner or later bring it to a halt. But wait, I'll show you something."
Alberto Knox got up and went over to the old desk. He took something out of one of the drawers. When he returned to his place he put it on the coffee table. It was just a wooden board, a few millimeters thick at one end and thin at the other. Beside the board, which almost covered the whole table, he laid a green marble.
"This is called an inclined plane," he said. "What do you think will happen if I let go the marble up here, where the plane is thickest?"
Sophie sighed resignedly.
"I bet you ten crowns it rolls down onto the table and ends on the floor."
"Let's see."
Alberto let go of the marble and it behaved exactly as Sophie had said. It rolled onto the table, over the tabletop, hit the floor with a little thud and finally bumped into the wall.
"Impressive," said Sophie.
"Yes, wasn't it! This was the kind of experiment Galileo did, you see."
"Was he really that stupid?"
"Patience! He wanted to investigate things with all his senses, so we have only just begun. Tell me first why the marble rolled down the inclined plane."
"It began to roll because it was heavy."
"All right. And what is weight actually, child?"
"That's a silly question."
"It's not a silly question if you can't answer it. Why did the marble roll onto the floor?"
"Because of gravity."
"Exactly--or gravitation, as we also say. Weight has something to do with gravity. That was the force that set the marble in motion."
Alberto had already picked the marble up from the floor. He stood bowed over the inclined plane with the marble again.
"Now I shall try to roll the marble across the plane," he said. "Watch carefully how it moves."
Sophie watched as the marble gradually curved away and was drawn down the incline.
"What happened?" asked Alberto.
"It rolled sloping because the board is sloping."
"Now I'm going to brush the marble with ink ... then perhaps we can study exactly what you mean by sloping."
He dug out an ink brush and painted the whole marble black. Then he rolled it again. Now Sophie could see exactly where on the plane the marble had rolled because it had left a black line on the board.
"How would you describe the marble's path?"
"It's curved ... it looks like part of a circle."
"Precisely."
Alberto looked up at her and raised his eyebrows.
"However, it is not quite a circle. This figure is called a parabola."
"That's fine with me."
"Ah, but why did the marble travel in precisely that way?"