* Velocity is a vector quantity: It has both magnitude (speed) and direction.
* Direction is changing: Even though you might be running at a constant speed of 5 km/h, your direction is constantly changing as you circle the track.
* Therefore, velocity is changing: Since velocity includes direction, and your direction is changing, your velocity is not constant.
Think of it this way: If you ran in a straight line at 5 km/h, your velocity would be constant. But running in a circle means you're constantly changing direction, so your velocity is changing even if your speed remains the same.
