ELI5: Explain Like I'm 5

Elementary Calculus: An Infinitesimal Approach

Okay kiddo, do you know what math is? It's like playing a game with numbers and symbols to figure out how things work in the world around us. One part of math is called calculus, which helps us understand how things change over time or space.

Now, when we talk about calculus, there are two types: differential calculus and integral calculus. Differential calculus is all about finding out how something changes, like how fast a car is going or how a plant grows. Integral calculus is all about finding out the total amount of something, like how much water flows through a pipe in a certain amount of time.

But today, we're going to talk about differential calculus and a special way of doing it called "infinitesimal approach." That's a big word, but all it means is that instead of looking at big changes, we're going to look at tiny, tiny changes called "infinitesimal."

Imagine you have a toy car and you want to know how much it moves over time. One way to do this is to measure the distance it travels in one second, two seconds, three seconds, and so on. But with calculus, we're going to do something different. Instead of measuring the whole distance, we're going to look at a tiny, tiny piece of the distance, like how much the car moves in one tiny fraction of a second. That's what we mean by "infinitesimal"- it's like a really, really small amount.

But how can we measure something so small? We use something called a "limit." A limit is like a rule that tells us what happens when we get closer and closer to a certain value. So for our toy car, if we want to know how fast it's going at a specific moment, we can use limits to figure out how much it moves over an infinitesimally small amount of time around that moment.

This may all sound really complicated, but it's actually really amazing! Infinitesimal calculus lets us understand how things change in a much more precise way. It's what helps engineers design cars that can go faster and doctors understand how the heart pumps blood through our body.

So there you have it, kiddo. Calculus is like a game that helps us understand how things change. And using an infinitesimal approach in calculus helps us measure super, super small changes that help us understand things even better.