External & Internal Forces
Summary
Internal forces are newton third law pairs inside a system
External forces are forces outside a system
Read Time
⏱ 1 min
Definition
External and internal forces are forces that act on a system.
Internal forces are forces on a system that act on one another. They are Newton's third law pairs, so the sum of internal forces is always zero and doesn't change the motion of a system.
External forces are forces outside a system, like gravity and friction. They don't come in Newton's third law pairs. They affect the system's motion.