INTRO: Conservation of Momentum is often violated in films.
Ice Age
Impossible Soap Bubble: Scrat
tries to catch up to Scratte by jumping inside his soap bubble and
hitting the top with his head from the inside to translate it vertically. This would not be
possible due to conservation of momentum.
Lock Stock and Two Smoking Barrels
Impossible Shotgun: Hatchet Harry shoots Dog with a shotgun, causing him
to fly back and out the door.
The momentum of the firearm projectiles is greatly over exaggerated.
Gravity
Imposible JetPack: George
Clooney uses his air powered jet pack to match orbital velocities with the
shuttle. Later Sandra Bullock uses a fire extinguisher to match orbital
velocities with the Chinese Space Station. The momentum of propellant is not
enough to bring the characters to sufficient velocities.
CONCLUSION: necessity, comedy, action.