My daughter wants to know, if she were not in orbit but traveling in a straight line in space (like a voyage to Mars), would she still experience weightlessness? We guessed not, because she wouldn’t be in free fall–it would be more like an airplane flight. Are we right?
That’s a good question. I’ve had to think about it too. In answer, I changed the question to, “What do you need to feel gravity, i.e., have a feeling of weight?” My reasoning: You need to be on a platform (something to stand or sit on) that is holding its position within a gravitational field. An airplane is holding its up/down position within the earth’s gravitational field, forward motion notwithstanding. Therefore, aboard the airplane you experience gravity much as you do on earth. Either that or you must be accelerating or decelerating so you feel the inertial force, which is indistinguishable from gravity.
Now, assume you have blasted off on a straight line mission to Mars. After gaining required speed, engines have been turned off; hence, you are neither accelerating nor decelerating so you feel no force from that. Nor is your rocket ship holding a position within a gravitational field. It may be traveling at great speed in a straight line, but it is not moving or holding a position counter to any gravitational field. Therefore, despite its straight line of travel, it and you in it are in a state of free fall within whatever gravitational field(s) are present. Therefore, you would feel weightless. Please ask further or give an alternative argument.