Eli5: When a super fast plane like blackbird is going in a straight line why isn’t it constantly gaining altitude as the earth slopes away from it?

Question

In a debate with someone who thinks the earth could be flat, not smart enough to despute a point they are making plz help.

In: 1372

Answer 1

Take a paper plate. Stick a thumb tack in the center.

Attach a length of string to the thumb tack. Pull the string tight. This is gravity.

Now move your hand forward. You’ll see that the string pulls your hand in a circle around the plate. This is because “gravity” is pulling you toward the plate and you follow the curve of the plate.