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

It takes energy to pick up an item off the ground. It takes energy to move an object further away from the Earth. If you only give the plane enough energy to stay at the same height that is what it is going to do. If you give it less energy from the engines it will eventually come down to the surface.

not Eli5…

If the plane was to go in an exact straight line and thus increase its distance from the center of the earth that would require added propulsion as the move away from the earth creates a greater gravitational potential. As in Newton’s first law there is a force (gravity) acting on the object so it does not stay in a straight line unless you give it enough force to increase altitude.