How far would an icicle have to fall to harm us?
An icicle’s piercing power depends on a few variables. The force of impact depends on the icicle’s momentum (mass multiplied by velocity). A higher drop means greater velocity as the icicle has more time to accelerate, but if it’s big enough, it will have adequate momentum even if it falls just a short distance. The sharpness of the icicle, meanwhile, dictates the concentrated force at the point of impact, which determines if it could pierce skin. The TV series MythBusters staged a plausible rooftop icicle incident scenario. In the experiment, they successfully pierced a steak with a 45-centimetre (17.7-inch) icicle falling from 4.6 metres (15 feet).
Answered by Tom Harris