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...