What’s Inertia? Ask Feynman!


He had taught me to notice things and one day when I was playing with what we call an express wagon, which is a little wagon which has a railing around it for children to play with that they can pull around. It had a ball in it—I remember this—it had a ball in it, and I pulled the wagon and I noticed something about the way the ball moved, so I went to my father and I said, “Say, Pop, I noticed something: When I pull the wagon the ball rolls to the back of the wagon, and when I’m pulling it along and I suddenly stop, the ball rolls to the front of the wagon,” and I says, “why is that?” And he said, “That nobody knows,” he said. “The general principle is that things that are moving try to keep on moving and things that are standing still tend to stand still unless you push on them hard.” And he says, “This tendency is called inertia but nobody knows why it’s true.” Now that’s a deep understanding—he doesn’t give me a name, he knew the difference between knowing the name of something and knowing something, which I learnt very early. He went on to say, “If you look close you’ll find the ball does not rush to the back of the wagon, but it’s the back of the wagon that you’re pulling against the ball; that the ball stands still or as a matter of fact from the friction starts to move forward really and doesn’t move back.” So I ran back to the little wagon and set the ball up again and pulled the wagon from under it and looking sideways and seeing indeed he was right—the ball never moved backwards in the wagon when I pulled the wagon forward. It moved backward relative to the wagon, but relative to the sidewalk it was moved forward a little bit, it’s just [that] the wagon caught up with it. So that’s the way I was educated by my father, with those kinds of examples and discussions, no pressure, just lovely interesting discussions.He had taught me to notice things and one day when I was playing with what we call an express wagon, which is a little wagon which has a railing around it for children to play with that they can pull around. It had a ball in it—I remember this—it had a ball in it, and I pulled the wagon and I noticed something about the way the ball moved, so I went to my father and I said, “Say, Pop, I noticed something: When I pull the wagon the ball rolls to the back of the wagon, and when I’m pulling it along and I suddenly stop, the ball rolls to the front of the wagon,” and I says, “why is that?” And he said, “That nobody knows,” he said. “The general principle is that things that are moving try to keep on moving and things that are standing still tend to stand still unless you push on them hard.” And he says, “This tendency is called inertia but nobody knows why it’s true.” Now that’s a deep understanding—he doesn’t give me a name, he knew the difference between knowing the name of something and knowing something, which I learnt very early. He went on to say, “If you look close you’ll find the ball does not rush to the back of the wagon, but it’s the back of the wagon that you’re pulling against the ball; that the ball stands still or as a matter of fact from the friction starts to move forward really and doesn’t move back.” So I ran back to the little wagon and set the ball up again and pulled the wagon from under it and looking sideways and seeing indeed he was right—the ball never moved backwards in the wagon when I pulled the wagon forward. It moved backward relative to the wagon, but relative to the sidewalk it was moved forward a little bit, it’s just [that] the wagon caught up with it. So that’s the way I was educated by my father, with those kinds of examples and discussions, no pressure, just lovely interesting discussions.

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