Imagine two people standing 10 feet apart – each throwing a ball to the other at the same speed of 10 feet/second, releasing the ball, at the same height, horizontally at the same time. How long before the balls meet and where do they meet?
Now imagine one person is 10 feet above the other (leaning out of a window perhaps!). They throw the ball to each other at 10 feet/second – one upwards and one downwards. How long before the balls meet and where do they meet?