Newton's first law states that a net external force causes a change in motion. External forces act on an object or system, originating outside of the object or system. In contrast, internal forces originate inside the system of interest and do not lead to any acceleration. In simpler words, internal forces are forces that act on one part of an object and are exerted by another part of the same object. External forces are forces that act on an object due to some other object. Therefore, when studying the motion of an object, we ignore the internal forces and consider only the external forces that lead to its acceleration. External forces generally include the applied force, normal force, tension force, friction force, and air resistance force. On the other hand, the internal forces include gravitational force, magnetic force, electric force, and spring force.

Consider two students pushing a stalled car with a driver inside. The system of interest is the car with the person inside it. The force exerted by the two students is an external force. In contrast, an internal force acts between elements of the system. The force the driver exerts to hold the steering wheel is an internal force between elements of the system of interest. Here, different forces exerted on the same mass produce different accelerations. The friction acts on the car, and it opposes the car's motion. A larger net external force produces a larger acceleration. Acceleration would then be directly proportional to and in the same direction as the net external force acting on the system.

This text is adapted from Openstax, University Physics Volume 1, Section 5.3: Newton’s Second Law.