Water Hammer

At the beginning of this semester I moved into a new apartment. Over the past couple of weeks I noticed that some of the plumbing makes a rattling noise when you shut a water valve off (i.e. turn the sink off, flush the toilet, turn the washing machine off). I was mindful that there had to be a fluid mechanics explanation as to why this is happening. Come to find out, this is a very common problem called water hammer. This occurs when you suddenly stop the water flow on either a hot or cold water line. The water lines in my apartment are pressurized so that when you open a valve a steady flow of water comes out. When you close a valve the water that was once running suddenly meets 100% resistance while slamming into the newly closed valve. The energy due to a momentary extra pressure  has no place to go but back up the pipe. When this extra energy runs back up the pipe is is slowly dissipated and if your plumbing is not secured properly this energy manifests itself in the form of vibrating your plumbing.