You realize that based on his logic, you proved his point? Lol. He's saying that if the earth was spinning, that knife should hit the back window. If it doesn't, it supports his theory. It's then up to you to provide a counterpoint as to why his theory is incorrect.
No he is saying that if the earth is moving he would land in a different spot because he goes up and the earth keeps going. Same with the knife. If they are moving and the knife goes up, the truck stays going and the knife hits the window. His example proved that the object going up doesn't stay in place while the truck/earth moves. It all still moves together.
His friend's theory is that IF the earth was spinning, which he doesn't believe it is, the knife would land in a different spot. Since the knife doesn't land in a different spot, he's providing support to his theory of the earth not spinning. You have to provide the answer as to why his theory is incorrect about something landing in a different spot. Just showing him that it lands in the same place supports his theory of the earth not spinning.
