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.
Read the full explanation
Understanding External Force
Imagine you’re drawing a circle around an object—that circle marks your 'system.' Any force that comes from outside the circle, like a push from another object or gravity from Earth, is an external force. Forces that occur inside the circle, like two internal gears turning each other, are internal forces and cannot change the system’s overall motion. To analyze how an object moves or stays still, you only care about the net external force. For example, when you kick a ball, your foot exerts an external force that accelerates the ball. If you and a friend push a box from opposite sides with equal strength, the two external forces cancel, and the box doesn’t move.
A deeper explanation
The reason external forces are so important lies in Newton’s second law: Fnet = ma. The net force in this equation is always the sum of external forces only. Internal forces come in equal and opposite pairs (Newton’s third law) and cancel out when considering the whole system. This principle explains why a rocket works: the thrust from exhaust gas is an internal force within the rocket-fuel system, but once the gas leaves the rocket, it becomes external to the remaining rocket, causing acceleration. This concept also underpins conservation of momentum—without an external force, total momentum of a system stays constant. Engineers use free-body diagrams to isolate an object and list all external forces acting on it, from gravity to friction to tension, enabling precise predictions of motion and equilibrium.