Astronauts and spacecraft are subject to both the force of gravity and "G forces." Although they are related, these forces are not necessarily the same thing. However, to understand G forces it helps to know something about gravitational force—the force that determines the motion of a planet around a star, the orbit of a satellite, or the motion of clusters of galaxies. In the presence of any massive object, such as a planet or star, any other mass experiences a force of attraction called gravitational force. This gravitational force is strictly proportional to the object's mass and the gravitational field, as in the formula F = m · g, where g is the gravitational field at any given location, and g exerts a force F on the mass m. The force F is also considered the object's weight.
At different points in space, the gravitational field generally has a different magnitude and direction. Therefore, the gravitational force acting on an object (its weight) changes as well. Newton's law of gravitation states that the gravitational force that two objects exert on one another also depends on their masses. This explains why astronauts on the Moon, which is much less massive than Earth, weigh only one-sixth as much as they do on Earth.
Besides being called the gravitational field, g is also considered the acceleration due to gravity. In fact, Newton's second law says that the force on an object is strictly related to the object's mass and acceleration—any type of acceleration. This means that if an object is accelerated it will experience G forces regardless of the gravitation force acting upon it. In practice, the term "G force" measures the magnitude of force due to nongravitational accelerations and represents the force of acceleration that pull on an object when it changes its plane of motion. Objects that are decelerated experience negative G forces.
Although G forces and the force of gravity are not synonymous, the force of gravity on Earth is used as a baseline for measuring G forces from acceleration or deceleration. When a person is simply sitting down, the force pressing her or him against the seat is the force of gravity. The intensity of this force is said to be "1G." The G force increases, however, if an astronaut is in a spacecraft that is accelerated. As the astronaut pulls more Gs, her or him weight increases correspondingly. An 80-kilogram (176-pound) astronaut in the space shuttle can experience 3Gs or more during liftoff, and her or him weight would thereby increase to 240 kilograms (528 pounds).
An astronaut in an orbiting spacecraft experiences weightlessness (often mistakenly call zero gravity). The cause of weightlessness is not the absence of gravity because gravitational force is still present. But the gravitational force is exactly balanced by the centrifugal force of the orbital trajectory, so that the astronaut is pulled with equal but opposite acceleratory forces that cancel each other out. For this reason, the astronaut floats in a state of weightlessness.
see also Flight Control (volume 3); Gravity (volume 2); Microgravity (volume 3); Rocket Engines (volume 1); Rockets (volume 3); Solid Rocket Boosters (volume 3); Zero Gravity (volume 3).
John F. Kross
Guyton, Arthur C., and John E. Hall. Textbook of Medical Physiology, 9th ed. Philadelphia: W. B. Saunders, 1996.
——. Human Physiology and Mechanisms of Disease, 6th ed. Philadelphia: W. B. Saunders, 1997.
Tilley, Donald E., and Walter Thumm. Physics. Menlo Park, CA: Cummings Publishing, 1974.