In relativity example , a time measuring device is shown in which a ball moves up and down. This device is then placed in a moving frame and the motion of the ball is observed from the ground. To an observer on the ground, the path followed by the ball will be a slant line. And therefore when it strike the top, it has to travel a greater distance and therefore it takes more time and the vertical velocity of the ball also decreases to the observer on the ground. Which is the reason why time run slower in a moving frame. My question why the vertical velocity of the ball decreases and the velocity of the ball on the path as observed by the observer on the ground doesn't change? Plz correct where i'm wrong!