An object’s weight, henceforth called “actual weight”, is the force exerted upon it by a gravitational field. By contrast, an object’s apparent weight is the normal force exerted on it by a body with which it is in contact. This will only equal the object’s actual weight if it holds the object at rest or at constant velocity (i.e. the vector sum of the forces acting upon the object is equal to zero).
An object’s apparent weight is equal to its actual weight, unless:
•The object has an acceleration, as in a lift, a rocket, or a rollercoaster.
•Some force other than gravity and the associated normal force is acting on the object. This may, for example, be buoyancy, centripetal force due to the Earth’s rotation, magnetic force.
Apparent weight is responsible for our sensation of the weight of our own bodies. A greater apparent weight results in a heavier or greater sensation of our weight, and vice-versa
Weightlessness is a phenomenon experienced by people during free-fall. The term zero gravity is often used as a synonym. Weightlessness in orbit is not the result of the force of gravity being eliminated, or even significantly reduced, by distance (in fact, the influence of the Earth’s gravity at an altitude of 200 km is only 6% less than at the Earths surface). Rather, the loss of the influence of gravity is due to the inertial motion of the flight path.
It is helpful to realize that (to an approximation limited by tidal forces) gravity cannot be felt as a force, by either objects or persons. Only the forces that resist gravity, or act apart from it, can be felt by people, or measured by accelerometers. These other forces (such as the force of the ground pushing upward on the feet) are those that produce the sensation and force of weight. Objects following inertial paths do not feel these other forces, and thus feel no g-force, and thus experience weightlessness.
Weightlessness, the sensation of feeling no forces, typically occurs when an object or person is falling freely, in orbit, in deep space (far from a planet, star, or other massive body), in an airplane following a particular parabolic flight path (e.g., the Vomit Comet), or in one of several other more unusual situations.