Explanation:

- Inertia: The tendency of an object to remain in its state of motion unless acted upon by another force. Not directly responsible for weightlessness.

- Zero Gravity: Often used to describe the condition of weightlessness. However, gravity is not zero; it's just countered by the spacecraft's free-fall.

- Centre of Gravity: The point in a body about which gravity acts. It doesn't cause weightlessness.

- Acceleration: Weightlessness in orbit results from continuous free-fall towards Earth. The spacecraft and its occupants are accelerating together due to gravity.