Final answer:
The magnitude of the ball's horizontal velocity remains constant while the ball is in the air if air resistance is ignored, because gravity only affects the vertical motion of the ball.
Step-by-step explanation:
When a ball is thrown at an angle of 50° to the horizontal, the magnitude of the ball's horizontal velocity remains constant during the time interval that the ball is in the air, provided we are ignoring air resistance. The reason for this is that the horizontal motion is not affected by gravity, which acts vertically downward. Thus, the initial horizontal component of velocity that the ball has when it is thrown is maintained throughout the trajectory.
Examples that illustrate this concept include a ball being thrown horizontally from the top of a building where it maintains its horizontal velocity or a projectile breaking into parts at the highest point of its flight where the forward motion continues unaffected by the event.