Work Step by Step
There is a nonzero gravitational force acting on the ISS and its inhabitants, but it is precisely this force that keeps it moving in a circle instead of flying off in a straight line. The ISS and the astronauts stay in orbit because they have a horizontal velocity. They fall toward the center of the Earth, but because they are also moving “sideways”, the Earth’ surface curves away from them at just the correct rate so that they stay at the same height above the surface. Being in freefall, the astronauts do not feel a normal force from the floor, i.e., their apparent weight is zero.