Picture a massive 50-ton tank rapidly closing in on your defensive position. Its tremendous momentum means regular obstacles won't cut it - you need something with sheer mass to halt the tank in its tracks. Your engineers' solution? A special cubic "hedgehog" obstacle constructed from dense steel plates. But to work, this metal cube must have enough mass to match the onrushing tank's - not too little to be plowed through, but not excessively over-built. The key challenge is calculating the minimum side length required for the steel cube to provide just the right mass to stop the 50-ton, 30 km/h tank through momentum transfer on impact. Get the dimensions wrong, and the tank will either smash through or you'll waste precious resources on an overly massive hedgehog. But if your calculations are accurate, your engineered metal cube will be a battle-winning roadblock against the unstoppable armored beast.