External forces affect the motion of the center of mass. Internal forces do not.

For the same force, an object with smaller mass will have a greater acceleration than an object with larger mass.

Net force and acceleration are directly proportional. Mass and acceleration are inversely proportional.

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass: $\vec{a}_{sys} = \frac{\sum \vec{F}}{m_{sys}}$

The vector sum of all forces acting on an object or system. It determines the acceleration of the object.

The rate of change of velocity of an object, directly proportional to the net force and inversely proportional to the mass.

When the net force acting on a system is not zero, causing the system to accelerate in the direction of the net force.

The sum of all forces acting on a system from outside the system, determining if the velocity of the system's center of mass will change.

Balanced forces: Net force is zero, constant velocity. Unbalanced forces: Net force is non-zero, acceleration occurs.

External forces: Act on the system from outside, can change the system's momentum. Internal forces: Act within the system, do not change the system's momentum.

Doubling net force: Doubles acceleration. Doubling mass: Halves acceleration.

Zero net force: Constant velocity (or at rest). Non-zero net force: Acceleration.

Net Force: Direction of the net force can be in any direction. Acceleration: Acceleration is always in the same direction as the net force.