Finally, science explains why no one can lift Thor's hammer

In the Avengers: Age of Ultron clip, Tony Stark speculates that there is a biosensor in the hammer’s shaft that recognizes when Thor has grasped Mjolnir. He is correct, in a sense—though it is not Thor’s fingerprints that the hammer is reading. Most likely it is taking some complex biological and psychological profile that calculates the “worthiness” of whoever is trying to lift the hammer. This is consistent with the scene in the clip where Steve Rogers (Captain America) is able to move the hammer (albeit slightly), while Tony Stark and Jim Rhodes, using thruster-assisted Iron Man and Iron Patriot gloves, are unable to budge Mjolnir at all. But if someone the hammer’s nanotechnology has determined to be “unworthy” tries to raise Mjolnir, how does it prevent itself from being moved?

Here the answer lies with Newton’s First Law of Motion, which states that an object at rest will remain at rest, if no net force acts upon it. The key word in the last sentence is “net.” When the hammer rests on the coffee table, there is a downward force on it from the gravitational attraction between the mass of the hammer and the mass of the Earth (which we refer to as its “gravitational weight”) and a counter-force from the book and tabletop pushing up on the hammer. This counter-force, referred to in physics as a Normal force (as it acts perpendicular or normal to any surface) is fundamentally electrostatic in nature, and is easy to take for granted except when it fails (as when one puts a thousand ton object on a table that can only provide a counter force of several hundred pounds).