The April 7 terror attack in Stockholm became more dangerous due to roadblockers in the Drottninggatan street. A strike by a 12000kg truck at 70km/h may eject an unrestrained 1000kg concrete block at 100km/h.
Well anchored stationary devices may stop the truck chassis, but if the barrier is too stiff, truck parts will become separate projectiles threatening people downstreams, like in the video below.
Seemingly unaware of Newton’s laws of motion, German researchers carried out full scale crash tests with a truck colliding with concrete blocks. Not surprisingly they found that “the anti-terrorist barriers were pushed like billiard balls” (quote with video, Daily Express Apr 12, 2017).
If such a “billiard ball” with a mass of 1000 kilograms is hit centrally by a 12000 kilogram truck at 70 kilometers per hour, simplified calculations indicate that it may be pushed away with a speed between 65 and 130 kilometers per hour.
The truck speed will be reduced to 65 km/h after an inelastic impact (the truck and the concrete block move together) and to 59 km/h after a fully elastic impact (increasing the block speed to 130 km/h).
Ground friction forces and vehicle lifting effects will reduce the speeds.
Do you want to make the calculations yourself? You may use your own numbers in the equations according to the YouTube video clip below:
This blog post was hastily prepared after my discovery on April 29 of the international unawareness of Newtonian mechanics.
I intend to post more info in due course with access to:
a) Downloadable Excel sheets, containing formulas for my calculations.
b) More links to video clips, demonstrating retractable net barriers etc.
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Form posted in the right hand column for desktops
(in the bottom menu for mobiles).
Professor Emeritus of Traveller Safety