Final answer:
Newton's second law of motion states that the acceleration of a system is directly proportional to the net external force acting on the system, and inversely proportional to its mass.
Step-by-step explanation:
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. This law can be expressed mathematically as F = ma, where F is the net external force, m is the mass of the object, and a is the acceleration.