What are inertia and momentum?

The idea about inertia applies to movable things like balls and bats and cars and planes. Anything that is at rest stays at rest unless something else gives it a push. Then it is a moving body. And when in motion, it tends to keep moving in a straight line unless some force pushes or pulls to stop it or make it turn.

So the idea about inertia is simply that something tends to keep on doing whatever it is doing.

When something is moving it has momentum. The heavier it is and the faster it is moving, the harder it is to stop. So momentum is a kind of measurement of how hard it is to stop something in motion.

In everyday life there is always some resistance to movement, which we call friction. Friction is like a drag or a pull against the direction of motion. This always works to slow down something in motion and make it come to rest again.

It always sounds complicated when we try to talk about things in very general terms. The best way to understand these ideas is to watch how movable things behave.