> [!summary] Internal forces are newton third law pairs inside a system External forces are forces outside a system >[!info]+ 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 Laws|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. # Resources <iframe width="560" height="315" src="https://www.youtube.com/embed/aAybYawUPC4?si=nyvXWRv8Z9tK_f0j" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>