Understanding Movie Physics: Apollo 13

Apollo 13 and the Definition of Weightlessness

Weightlessness seems like it has a pretty straight-forward definition, but to be honest it isn't as simple as it sounds!

In many contemporary space faring movies, such as "2001: A Space Odyssey" or in our case "Apollo 13",  the characters seems to be experiencing "zero gravity" or "weightlessness".  It is a common held notion that out in the depths of space there is no force of gravity to act on the objects floating around out there, but scientists have figured out that such a thing is impossible. It is what keeps all asteroids and planetary bodies in orbit. The gravity of a star also has incredible reach as asteroids even beyond Pluto stay in orbit around the sun. It turns out that the force of gravity can get incredibly close to zero but never reach it and that the only way for there to be total zero gravity is to get infinitely faraway from any other object of mass. 

This is where the idea of weight comes in.  The weight of an object is dependent on that object's mass and its relationship with the gravity of the planetary body that object is on or around. However what humans perceive as weight is actually a force that we call, a normal force of an object. The normal force is a force that resists the force of gravity as it is an "opposite reaction" to the initial action that is gravity, meaning that it has the same amount of force but an opposite direction and its this resistance that we perceive as weight. And the lack of this normal force is what helps define weightlessness.

The normal force of an object can only be removed if an object is sufficiently removed from a planetary that it is considered to be in free fall. Free fall is defined as a situation where the only force acting on an object is gravity. An example of this are astronauts on the ISS (which actually experiencing a very long or extended free fall but is going at fast enough speed so that it doesn't plummet to the Earth)) or for a film example is any scene in 'Apollo 13" where they are floating in their spaceship between the Earth and the Moon where the gravity is weak but strong enough to still affect them.

Therefore, in order to conclude or summarize, the only way to experience "weightlessness" is to remove the normal force of an object by putting it in free fall so the only force acting on it is gravity and that the object is at a sufficient distance away from a planetary body so that its gravitational pull is relatively small and is going at a fast enough speed to keep it from falling.

I rate "Apollo 13" a PGP level movie.