In order for games to feel natural, the physics of a game should match the physics of the real world. Most engines provide a library to handle physics. Unity provides the Physics class that contains physics properties such as gravity along with helper methods.
Defining Velocity
Velocity is the change in position divided by the change in time. The change in position is called the displacement. Acceleration is the change in velocity divided by the change in time. Both velocity and acceleration are represented as vectors. The magnitude of the vector represents the speed.
Unity uses meters per second so an x coordinate of 2 would mean 2 m/s on the x axis. It is important to understand where velocity is being calculated. For instance, if using in the Update method which is called per frame, the velocity vector must be multiplied by Time.deltaTime in order to change the units from per second to per frame.