Physics
External Force
Quick fact
If no external force acts on an object, its velocity remains constant—even if it's at rest. This is Newton's first law, and it means a ball rolling on a perfect frictionless surface would never stop.
Why this is interesting
You’re in a car that suddenly brakes—your body lurches forward. What invisible hand pushes you? That push is an external force from the seat belt, and it’s the only reason you don’t keep moving forward forever.