Why is Inertia Important in Physics?

[GrizzlyBat]

Hey, I was just wondering what Inertia is used for? I mean, I am not talking about Rotation Inertia or anything like that. I am wondering what the term means "when something resists movement". I have a feeling that it is an old term that people stopped using, but why?

 

[mikelepore]

Push a shopping cart. You can see how much it accelerates. Remember how hard you pushed. Now go over to a train full of elephants and push it just that hard. It doesn't accelerate nearly as much. What the train has more of, that the shopping cart has less of, that's inertia. You can't call it its weight because you'd get the same result if you were floating in space far from any world's gravity. Why do most people not need the word "inertia"? Because it _proportional_ to weight, so in ordinary conversation most people just call it the weight. People won't learn a word unless they find it helpful for something.