When calculating acceleration due to a force, we consider only the change in velocity of an object because acceleration is defined as the rate of change of velocity. It measures how quickly an object's velocity is changing over time.
Velocity is a vector quantity that includes both magnitude (speed) and direction. When a force is applied to an object, it can cause a change in either the magnitude, the direction, or both of its velocity. The acceleration represents how much the velocity of the object changes per unit of time.
By focusing on the change in velocity, we can quantify the effect of the force on the object's motion. The acceleration provides information about the rate at which the object's velocity is changing, regardless of whether the change is due to a change in speed, change in direction, or a combination of both.
It's worth noting that the force applied to an object can indeed have various effects on its velocity. For example, if the force is applied in the same direction as the object's motion, it can increase its speed. If the force acts in the opposite direction, it can decrease the speed. If the force acts perpendicular to the velocity, it can change the direction of motion without affecting the speed.
Considering only the change in velocity allows us to focus on the net effect of all the forces acting on the object and measure the resulting acceleration. This simplification enables us to study and analyze the motion of objects under the influence of various forces more effectively.