Inertia explains why motion does not need a continuous push
An object keeps its state of motion unless a net external force changes it.
Takeaway
Forces explain changes in motion, not motion itself.
What I learned
Newton's first law says an object at rest stays at rest, and an object in motion continues at constant velocity, unless acted on by a net external force.
Why it matters
This idea is unintuitive because everyday motion is full of friction. A rolling ball slows down not because motion naturally runs out, but because external forces act on it.
How I can use it
When analyzing motion, first ask what is changing. Then look for the force causing that change.