How can acceleration change the motion of an object?

It has the ability to alter the velocity vector's size, direction, or both.

When an object accelerates, its velocity changes in most cases, either increasing or decreasing depending on whether the acceleration is moving in the same direction as the velocity vector or in the opposite direction. If the acceleration is moving in the same direction as the velocity, the velocity will increase, and if it is moving in the opposite direction, the velocity will decrease.

In one-dimensional problems, the acceleration solely influences the velocity vector's size; the direction remains unchanged, so the only thing to consider is whether the acceleration is moving in the same direction or in the opposite direction as the velocity. In two- or three-dimensional problems, however, the velocity vector's size and direction may vary, necessitating the application of mathematical techniques to determine the resulting velocity vector. Projectile motion is one example of this.

A good example of this is circular motion with constant speed, where the acceleration acts towards the center of the circle and the velocity vector is always tangential to the circular path, meaning they are always perpendicular. (Hence the speed remains constant because the size of the velocity vector does not change.) In some cases, the acceleration is perpendicular to the velocity vector. In these cases, the acceleration will change the direction of the velocity.

Acceleration can change the motion of an object by altering its velocity, either by speeding it up, slowing it down, or changing its direction.