Why is acceleration a vector quantity?

Acceleration is a vector quantity because it has both magnitude and direction.

It tells how quickly an object’s velocity changes and in which direction the change occurs. Since velocity itself is a vector (it depends on direction), the rate of its change must also depend on direction.

For example, when a car speeds up northward, its acceleration points north; when it slows down, acceleration points opposite to motion. The direction of acceleration determines whether the object speeds up, slows down, or changes direction. 

In equations, acceleration is shown as a = Δv / Δt, where Δv is the change in velocity vector. Thus, acceleration’s vector nature is essential in describing real motion accurately.