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?”
Sophie thought deeply. Then she said, “Because the board was sloping, the
marble was drawn toward the floor by the force of gravity.”-
“Yes, yes! This is nothing less than a sensation! Here I go, dragging a girl
who’s not yet fifteen up to my attic, and she realizes exactly the same thing Galileo
did after one single experiment!”
He clapped his hands. For a moment Sophie was afraid he had gone mad. He
continued: “You saw what happened when two forces worked simultaneously on the