失重

Weightlessness is a sensation felt when all contact forces acting on a body disappear, leaving only gravity. Astronauts in a spacecraft accelerating downward at gravitational acceleration g experience weightlessness because no reaction force acts on them. Although gravity exists everywhere in the universe, astronauts do not feel its effects during free fall, making their apparent weight zero.

All bodies fall with constant acceleration due to gravity when air resistance is ignored. In free fall, the only force acting on a body is its weight, with no contact forces present. Since all objects accelerate at the same rate regardless of mass, they all experience weightlessness simultaneously, reaching the ground at the same time.

At the top of a roller coaster loop, the rider is momentarily lifted off the seat, removing the normal force. With only gravitational force acting on the rider, they feel weightless despite gravity still pulling them downward. This sensation occurs because the absence of contact forces creates the perception of zero weight, even though the rider's actual weight remains unchanged.

A weighing scale would read zero because no reaction force acts on the person during free fall. The scale measures the normal force between the person and the platform, which disappears when both fall together at gravitational acceleration. This demonstrates that weightlessness is a sensation caused by the absence of contact forces, not the absence of gravity itself.

No. Weightlessness is merely a sensation caused by the absence of contact forces, not the absence of actual weight or gravity. An object's weight always acts when on Earth because gravity continuously attracts it toward the center. Weightlessness occurs when internal and external forces balance such that no normal force opposes gravitational acceleration.

When air resistance is ignored, all bodies fall with constant gravitational acceleration and experience weightlessness. However, objects falling on Earth never experience true free fall because air always exerts an upward resistance force. This air resistance reduces the net downward force, preventing the object from accelerating purely under gravity alone.

Free fall occurs when the only force acting on an object is gravity, with no contact forces present. During free fall, an object experiences weightlessness because the absence of normal forces creates zero apparent weight. Whether in an aircraft or orbiting spacecraft, the sensation of weightlessness is identical when free fall occurs, demonstrating that weightlessness depends on force conditions, not location.