# Why is acceleration a vector quantity?

Think of a ball thrown up into the air. Gravity is accelerating the ball at a constant rate of $g = 9.8 \frac{m}{s} \left[\mathrm{do} w n\right]$ . When the ball is traveling upward, acceleration is in the opposite direction, and the ball slows. When the ball slows to a velocity of $0 \frac{m}{s}$, gravity is still acting upon the ball. Then the ball begins to move downward because gravity is still acting upon it, but now the motion and acceleration are in the same direction, so the ball speeds up.