Final answer:
An external force is a force that acts on a system from outside and causes a change in its angular motion.
Step-by-step explanation:
An external force is a force that acts on a system from outside the system. It is different from internal forces, which act between components within the system. In physics, Newton's second law of motion states that the acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system, and inversely proportional to its mass.
For example, if you push a cart, the force you exert on the cart is an external force that causes a change in the cart's angular motion. Similarly, if you throw a ball, the force you exert on the ball is an external force that changes its angular motion.
So, in summary, an external force is a force that acts on a system from outside and causes a change in its angular motion.