Question

What is inertia?

Answer 1

Inertia is the Isaac Newton's first law of motion. It states that an object in motion will remain in motion and an object at rest will remain at rest unless there is net force acting on the object. That basically means that anything that's moving will keep on moving and anything that isn't moving won't move unless something makes it.

For example, if a car is moving, it will continue moving until something stops it, like pressing the brakes to make it stop. And if it isn't moving, it will stay that way until something makes it, like if you start the car again.

Answer 2

Inertia is the resistance to change. All objects with mass have it. So if something is moving fast, it does not want to slow down on its own, unless something else slows it down, like gravity, friction, or another object hitting it. The opposite is true too. If an object is not moving, it does not want to move on its own unless some force acts on it.

I don't think anyone really knows why this is the case, but it just is and nobody has ever found it not to be true. If it was'nt for inernia, your pencil could just decide to move away on its own whenever it wanted, which would make for a very strange world.