A number of science fiction books and films have featured spacecraft that rotate very
slowly as they travel through the universe. Arthur C. Clarke’s science fiction novel
Rendezvous with Rama is set in a massive rotating spacecraft, as is part of the movie
2001 , which is also based on his work. This rotation supplies artificial gravityí the
illusion of gravity í to the astronauts and their equipment. Artificial gravity has effects
similar to true gravity, and as a result can mislead the people riding in such machines to
believe they are experiencing true gravitational force.
Why does the rotation of a spacecraft produce the sensation of gravity? Consider what
happens when an airplane takes off from a runway: You feel a force pulling you back
into your seat, as if the force of gravity were increasing. The force of gravity has not
been significantly altered (in fact, it decreases a bit as you gain elevation). However,
while the airplane accelerates upward, you feel a greater normal force pushing up from
your seat, and you may interpret this subconsciously as increased gravity.
A roughly analogous situation occurs on a rotating spaceship. The astronauts are
rotating in uniform circular motion. The outside wall of the station (the floor, from the
astronauts’ perspective) provides the centrally directed normal force that is the
centripetal force. This force keeps the astronauts moving in a circle. From the
astronauts’ perspective, this force is upwards, and they relate it to the upward normal
force of the ground they feel when standing on the Earth. On Earth, the normal force is
equal but opposite to the force of gravity. Because they typically associate the normal
force with gravity, the astronauts may erroneously perceive this force from the
spacecraft floor as being caused by some form of artificial or simulated gravity.
Artificial gravity is a pseudo, or fictitious, force. The astronauts assume it exists because
of the normal force. The perception of this fictitious force is a function of the
acceleration of the astronauts as they move in a circle. It would disappear if the
spacecraft stopped rotating.
Although discussed as the realm of science fiction, real-world carnival rides (like the
“Gravitron”) use this effect. Riders are placed next to the wall of a cylinder. The cylinder
then is rotated at a high speed and the floor (or seats) below the riders is lowered. The
walls of the cylinder supply a normal force and the force of friction keeps the riders from
slipping down.
Artificial gravity
Space station rotates
Floor of craft provides centripetal force
Person (incorrectly) assumes normal
force counters force of gravity