Final answer:
The acceleration of the center of mass of a system can only be caused by external forces, as internal forces cancel out. Only external forces can change the velocity or acceleration of the center of mass, while internal forces do not have any effect on it.
Step-by-step explanation:
In physics, the center of mass is a point that represents the average position of all the individual masses in a system. According to Newton's laws of motion, the acceleration of the center of mass of a system can only be caused by external forces. This is because all internal forces within the system cancel out and do not contribute to the overall motion of the center of mass.
For example, consider a system of two interacting objects. The internal forces between these objects, such as the gravitational attraction between them, will balance out. However, if there is an external force acting on the system, such as a push or a pull, it will cause the center of mass to accelerate in the direction of the force.
Therefore, only external forces can change the velocity or acceleration of the center of mass of a system, while internal forces do not have any effect on it.