Wrecking Ball Simulator

How the Wrecking Ball Simulator works

The wrecking ball is the image everyone has of demolition: a steel ball on a crane, swinging into a brick wall. Real ones typically weigh from a few hundred kilograms to more than five tonnes. They were a standard demolition tool for most of the twentieth century and are now mostly replaced by high-reach excavators, which bring buildings down more precisely and throw less debris into the street. The physics is still a good lesson in pendulums, momentum and why unmortared walls fall the way they do, and this simulator lets you knock one down as often as you like.

How the wrecking ball simulator works

The scene runs on the Rapier physics engine. The ball hangs from a chain of ten rigid capsule links joined end to end, so it swings in a real arc and can twist, wrap and recoil rather than sliding along a fixed rod. Each brick is a solid 0.6 m cube with its own weight and friction. The tower is hollow and has no mortar, so it stands up by gravity and friction alone and falls when the ball pushes enough of the lower rows out of place.

Press Swing and the ball gets an impulse towards the building. The impulse is scaled by the ball's mass, so each press adds the same speed whatever the ball weighs, equal to the Swing power setting in metres per second. Press it again on the return and the swings build up, just as pushing a child on a swing at the right moment does.

The settings and what they change

Ball weight has a bigger effect than it looks. The radius grows from 0.6 m to 1.5 m across the range and the density rises too. Mass is density times volume and volume goes with the cube of the radius, so the heaviest ball is roughly 150 times the mass of the lightest.

The physics behind the swing

A pendulum turns height into speed. A ball released from a height h above the bottom of its arc arrives at a speed of √(2gh), whatever its mass. From 3 m that is about 7.7 m/s. What mass changes is what happens at impact. Momentum is mass times velocity, and kinetic energy is ½mv², so a ball twice as heavy at the same speed carries twice the energy into the wall.

That is why a light ball bounces back off the same wall a heavy one ploughs through. When the ball's momentum is small compared with the bricks it hits, most of it goes into recoil. When it is large, the ball barely slows, keeps pushing, and the bricks behind the first layer go too.

Reading the stats

• Swings counts how many times you have pressed Swing since the last restart.
• Bricks knocked down shows fallen bricks against the total. A brick counts once it has dropped most of a row below where it started, so a brick that slides along the ground does not count, but one that falls from the top does.
• Tallest point is the height, in rows, of the highest brick still anywhere in the scene. A single brick lodged on the rubble still counts, so it can stay high after a collapse.
• Ball speed is the live speed of the ball in metres per second. It peaks at the bottom of each arc and drops to nearly zero at the top.

Tips for a clean demolition

• Hit low. Real demolition crews knocked out the lower storeys of a wall so the upper ones fell inward under their own weight. The simulator behaves the same way: the base carries the load, so a strike low down brings more of the tower with it.
• Time your presses. Press Swing as the ball starts moving towards the building again. Pushing against its motion cancels speed instead of adding it.
• Don't overdo the power. Too much speed sends the ball up and over a short tower. The chain then snaps taut and drags it back, and you have wasted a swing.
• Tall and thin versus short and wide. A 3-by-3 tower 20 rows high often folds from one strike to the base. A wide, low block loses bricks from the side facing the crane and keeps standing, which is why wide buildings took many swings.
• Try a light ball on a big wall. It is the quickest way to see momentum at work: it bounces off almost every time.

More physics toys

Swap the ball for a chain reaction with 3D domino toppling, or take the tower apart one block at a time in 3D Jenga. The 3D physics sandbox lets you drop and stack your own shapes, and Newton's cradle shows momentum passing through a row of balls.

Restart rebuilds the building and re-hangs the ball with the current settings. Everything runs locally in your browser; nothing is uploaded.

Related tools