Equalibrium 問題

In fact, the sapcecraft is NOT in equilibrium.

As you have siad that the spacecraft is in orbit revolving (do not use the word "rotating", which would mean "spinning" about an axis) round the earth, it is in a state of motion.

Although the speed of the spacecraft doesn't change, its velocity keeps changing. As such, there is a NET FORCE acting on the spacecraft, as a net force would lead to an acceleration (by Newton's Second Law), and then a change of velocity.

In this situation, the net force is given by the gravitational attractive force of the earth. This attractive force keeps the spacecraft continuously change its velocity and results in a circular orbit round the earth. Without the earth's attraction, the spacecraft would travel in a straight line insteard of a circular path.

When a spaceship is rotating around the earth, it experiences a centrifugal force (離心力) acting on it. This force is increased as the angular velocity of the spaceship increases.

When the angular velocity of the spaceship reaches a certain value, the centrifugal force would be equal (and in opposite direction) to the gravitational force (地心吸力) acting on the spaceship, i.e. no net force, the spaceship would keep on rotating around the earth, neither drop down to earth nor fly away into the space.

Equilibrium between the earth and the spaceship is established.