When an object is dropped, it accelerates toward the center of the Earth. If the net external force on the object is its weight, it is said to be in free fall; that is, the only force acting on the object is gravity. Galileo was instrumental in showing that, in the absence of air resistance, all objects fall with the same acceleration g. However, when objects on the Earth fall downward, they are never truly in free fall, because there is always some upward resistance force from the air acting on the object. Weightlessness, or zero gravity, is a sensation felt by a body when all contact forces acting on it disappear.

Technically speaking, gravity exists everywhere in the universe because it is defined as the force that attracts two bodies to each other. However, astronauts in space usually do not feel its effects. Astronauts in a spacecraft accelerating downward at g feel weightlessness. If they were to measure their weight during free fall using a weighing scale, the reading on the scale would be zero. There is no difference in the apparent weightlessness in the aircraft and in orbit when free fall is occurring. Consider a person standing on a weighing scale in an elevator during free fall. Here, since no reaction force is applied to them, they would experience weightlessness. Thus, the reading on the weight scale would be zero.

This text is adapted from Openstax, University Physics Volume 1, Section 5.4: Mass and Weight.