Water Hammer
Definition
The bang in pipes when a tap slams shut — the water column's momentum converts into a pressure spike.
Technical definition
Joukowsky's relation Δp = ρcΔv (c = wave speed in the pipe). Arrestors add air cushions to absorb the surge.
Example
The clunk when a washing-machine valve snaps shut is water hammer — plumbers install arrestors to protect joints from the repeated spikes.
Common confusion
Water hammer is not air trapped in the pipes — air actually softens it. It is the momentum of the water itself, stopped too abruptly.