Physics in 'Walking Dead'

  Oct.27.2016




Have you ever watched the "Walking Dead"? It is a famous TV show in America.
Portrays life in the weeks and months following a zombie apocalypse. Led by police officer Rick Grimes, his family and a group of other survivors find themselves constantly on the move in search of a safe and secure home. You can see many different survival skills in this show, many of them are related to basic physics.  Today, I'd like to introuduce one of them.

  In Episode 5, people found a fire truck when they were attacked by zombies. Then  a scientist used the fire truck water hose to fight them. He beat all zombies with water hose.  Zombie's heads were crushed by the water. How is this possible? How powerful is the fire truck water hose?

A fire hose is a high-pressure hose that carries water or other fire retardant (such as foam) to a fire to extinguish it. Outdoors, it attaches either to a fire engine or a fire hydrant. Indoors, it can permanently attach to a building's standpipe or plumbing system.
The usual working pressure of a firehose can vary between 8 and 20 bar (800 and 2,000 kPa; 116 and 290 psi) while its bursting pressure can be up to 83 bar (8,300 kPa; 1,204 psi).
It is more than pressure of  Natural gas pipelines: 800–1000 psig.


Can the water pressure from a fire hose powerful enough to stop a bullet?

 Yes, and you wouldn't need much water either, or for it to be moving very fast. Water is very good at slowing down very fast moving objects, much more so than air. The water that comes out of the hose may be moving fast for a person, 30-80 miles per hour, but this is very slow in comparison to the bullet, which will be travelling at ~880 mph. The speed of the bullet will vary far more than the speed of the water. So to estimate just look at how fast bullets are stopped by still water. A 9mm full metal jacket will only penetrate between 1 - 2 feet of water before coming to a rest.
So, if you are shooting the bullet into the on coming water from a fire truck the bullet will be easily stopped. However if you shoot at a 90 deg angle to the spray it will be deflected, but will still retain some of its momentum.
 For practical demos of similar things I suggest checking out box O'Truth. Which is just many tests of bullets vs. water and walls.
Based on what people said below I think the part of it not being all water is the closest. An assumption of 1-2 feet might be too optimistic. However I think you could use the radius² of the water where you hit the stream, vs the radius² of the water as it exits the hose as a good assumption of the water vs air ratio. This would require someone with a bit more knowledge of how water in a jet moves unconstrained though the air. However I think the point remains that the bullet would be easily stopped within a few feet, so long as it didn't leave the stream at a strange angle.




Joohee Lee