There are multiple inconsistencies with Ant-man's physics. It was specifically mentioned in the movie that his mass remains constant despite his small size, and here are instances where that idea was supported:
- While ant-sized, Ant-Man falls from the edge of bathtub onto a tile floor, cracking the tile as though with the weight of a full sized human.
- He falls from a rooftop onto a car, denting the roof.
- He punches full sized humans, and they respond as though hit by a full-sized punch.
- He's able to judo-flip full sized humans
- Another character, while tiny, is hit by a speeding toy train. The train is knocked aside as though it hit a full sized human.
But then there are times when their mass seems implied to be as light as their tiny size:
- Ant-Man is constantly running and climbing around on surfaces (air vents, ant tunnels, etc.) that couldn't possibly support the weight of a 200 lb man on a footprint the size of an ant.
- The toy train, when expanded to the size of a real train, is heavy enough to rip through a wall and crush a police car.
- Hank Pym carries a shrunken tank in his pocket on a keychain. It obviously doesn't weigh as much as a tank, but it does when expanded to full size.
- Another character, while tiny, gets swatted with a (normal sized) table tennis paddle, and goes flying like an insect.
- a flying ant is able to support Antman riding on it
- Antman riding on Hawkeye's arrow without affecting trajectory