Here's why:
* Acceleration is the rate of change of velocity. Velocity has both speed and direction.
* If acceleration is in the same direction as motion, it means the object is getting faster in that direction.
* Since speed is the magnitude of velocity, an increase in velocity directly translates to an increase in speed.
Example:
Imagine a car moving forward. If the car accelerates forward (meaning the acceleration is in the same direction as the car's motion), the car will go faster and faster.
