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Reminder to politicians: concrete blocks do not help against trucks

Posted by jpluimers on 2019/10/18

These physics lessens at school were useful after all.

TL;DR:

A concrete block of ~1.500kg will loose against a truck of ~9.000kg, especially if that drives 50 km/hour.

It will not stop the truck, but will start moving by itself in unexpected directions and speeds becoming a projectile by itself or worse: usually pieces break off traveling at quite high speed.

The video below shows what happens.

Beton blocks

Over the last year or so, concrete blocks are deployed in many places of the public areas. The usual deeper motivation is to protect against traffic.

The blocks are put on the ground without anchoring for a variety of motivations like flexibility, ease of deployment/removal, cost of blocks (EUR ~100 each) versus anchoring (EUR ~250 per block) in a non-interconnected way.

Often, the rectangular lego-like blocks with 8 bumps are used which come in two varieties: 40cm high (easier to sit on, look more friendly) of a mere 1200kg or 80cm high (look more massive) of only 2400kg.

Other concrete blocks used are roughly the same dimensions, so an average weight of ~1500kg is reasonable.

Trucks

An average truck (at about 10.000 kg) isn’t a static object. In cities they are usually allowed to drive at 50 km/hour, but during assassination attempts they drove much faster and also were much heavier.

Let’s assume however that a truck used is less heavy (not all bad people are smart to get a really heavy truck) at ~9.000kg.

Truck concrete collision

The assumptions so far: a truck of 9.000kg at 50 km/hour against a concrete block of 1.500kg at standstill.

Even though a collision with a truck looses some energy, a moving truck has a lot of it. So most of the energy from the truck will be partially or fully transferred via its momentum to the concrete block(s).

The physics involved here are about momentum:

  • momentum = mass * velocity
  • momentum in a system is conserved

Before colliding, the truck has momentum, but the concrete block does not. After the collision, the momentum is divided over truck and concrete block so they both have a velocity.

A few cases that can happen, usually in a combined fashion:

  1. The truck comes to a full stop and all momentum is transferred to the block. The block now travels 9.000/1.500 * 50 km/h which is 200 km/h if it was in front of the truck.
  2. If the concrete wasn’t fully in front of the truck, the truck will move in another direction as well as the concrete block. Those directions are hard to predict for the public.
  3. Part of the concrete comes off during collision. Since their weight is smaller, their speed will be higher (because momentum is conserved) and direction even less predictable.

–jeroen

References

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