The Physics of Fireworks

Kids and adults alike adore fireworks, but have you ever thought about how they work? Whether it's the Fourth of July, a weekend at the lake or a special celebration in your community, you can use fireworks to explain the laws of physics to your child.

Fighting Gravity
Gravity is the force that keeps everything attached to the Earth. Without it, we'd all float off into space. Those high-flying fireworks need to overcome the force of gravity, and the people who make fireworks and stage shows use a precise amount of fuel to ensure that fireworks explode safely overhead, rather than on the ground.

The displays seen at public events use fireworks with at least two elements. One is a shell that explodes in colors or crackles. The other is a small rocket engine. To launch fireworks into the air, they're loaded into metal or cardboard tubes. When the fuse is lit, materials in the rocket part of the shell burn very quickly. If these materials were spread out on the ground, they'd burn up immediately in a flash, which wouldn't be very exciting.

Because they're burning in a confined space, the launch tube, the rocket engine builds pressure beneath it, which is known as thrust. So much energy builds up that the firework lifts out of the tube and into the sky. Once it's there, inertia, or the tendency of an object to keep moving, works with the rocket engine to keep it flying higher. A separate fuse inside the firework ignites the shell when it reaches the top of its trajectory. Eventually, gravity pulls the ash and the remnants of the shell back to Earth.

Creating Different Shapes
A chrysanthemum shell is one of the simplest fireworks to make, because it operates on a simple rule of physics: explosive force travels equally in all directions from the point of ignition. In other words, if something blows up in the air, where there's little resistance, it will create a sphere. If that same explosion took place on the ground, you'd wind up with a dome-shaped blast, since objects in motion will travel farther if there's little resistance. You'd still have a crater at the site of the blast, but most of the energy would reflect upward.

Fireworks makers use the laws of physics to create the different shapes that we see during displays. A chrysanthemum has a weak outer wrapper that blows apart in all directions, creating a sphere. Palms, which are larger on the top, are held in reinforced tubes that push most of the blast to the top, creating an effect that looks like a palm tree in the sky.

A Hole World of Difference
Most people are familiar with Jumping Jacks and firecrackers. These are the small fireworks that people buy for backyard Fourth of July celebrations.

If you put a Jumping Jack next to a firecracker, they look almost identical. Both are small, tightly wrapped tubes of paper with a fuse in one end and flammable powder inside. When you light a firecracker, you get a loud bang. When you light a Jumping Jack, it spins and bounces on the ground and occasionally takes off into the air. Why does this happen?

Look closely at that Jumping Jack and you'll see a tiny hole punched into one end. This it what causes it to move around. When the powder inside ignites, it creates a tiny jet of flame that propels the firework across the ground. The location of the hole near the bottom, coupled with the light weight of the firework, causes it to bounce in a rotating, triangular pattern.

The firecracker doesn't have a hole, so there's no place for energy and heat to escape when its powder burns. The pressure builds up inside the firecracker until it explodes with a loud bang. When matter changes from one state to another, it releases gases and energy. If these gases and energy don't have a way to safely disperse, the result is an explosion.

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