People rarely get to experience firsthand the phenomenon of weightlessness, but that doesn’t keep SAT II Physics from testing you on it. There is a popular misconception that astronauts in satellites experience weightlessness because they are beyond the reach of the Earth’s gravitational pull. If you already know this isn’t the case, you’re in a good position to answer correctly anything SAT II Physics may ask about weightlessness.

In order to understand how weightlessness works, let’s look at the familiar experience of gaining and losing weight in an elevator. Suppose you bring a bathroom scale into the elevator with you to measure your weight.

When the elevator is at rest, the scale will read your usual weight, W = mg, where m is your mass. When the elevator rises with an acceleration of g, you will be distressed to read that your weight is now m(g + g) = 2mg. If the elevator cable is cut so that the elevator falls freely with an acceleration of –g, then your weight will be m(g – g) = 0.

While in free fall in the elevator, if you were to take a pen out of your pocket and “drop” it, it would just hover in the air next to you. You, the pen, and the elevator are all falling at the same rate, so you are all motionless relative to one another. When objects are in free fall, we say that they experience weightlessness. You’ve probably seen images of astronauts floating about in space shuttles. This is not because they are free from the Earth’s gravitational pull. Rather, their space shuttle is in orbit about the Earth, meaning that it is in a perpetual free fall. Because they are in free fall, the astronauts, like you in your falling elevator, experience weightlessness.

Weightless environments provide an interesting context for testing Newton’s Laws. Newton’s First Law tells us that objects maintain a constant velocity in the absence of a net force, but we’re so used to being in an environment with gravity and friction that we never really see this law working to its full effect. Astronauts, on the other hand, have ample opportunity to play around with the First Law. For example, say that a weightless astronaut is eating lunch as he orbits the Earth in the space station. If the astronaut releases his grasp on a tasty dehydrated strawberry, then the berry, like your pen, floats in midair exactly where it was “dropped.” The force of gravity exerted by the Earth on the strawberry causes the strawberry to move in the same path as the spaceship. There is no relative motion between the astronaut and the berry unless the astronaut, or something else in the spaceship, exerts a net force on the berry.