Why do some explosions shake cameras before the shockwave reaches it?


I was scrolling through YouTube when I found a video of the 2020 Beirut explosion. (I wouldn’t have clicked on it had it not been for the fact that it was an angle I hadn’t seen before.) I noticed that as the final ammonium nitrate explosion occurred, the camera was moved and a whoosh was heard before the actual shockwave and boom reached the camera. Shockwaves travels at the speed of sound and this was what was visible as the massive condensation cloud, so what was this force that moved the camera almost as soon as the explosion began?

In: 4

When an explosion occurs, it generates multiple forms of energy, including heat, light, sound, and pressure waves. The pressure wave is what creates the shockwave that we see and hear as the explosion propagates through the air.

However, before the pressure wave reaches a camera, it can sometimes be affected by the initial blast wave, which is a high-speed wave of hot gas and debris that is expelled from the explosion site. This blast wave can move quickly and can cause objects, such as cameras, to vibrate or shake. This is what you observed in the video of the Beirut explosion.

Depending on how the camera is mounted it was probably earth shock. Explosive energy doesn’t just travel through the air. It also enters the ground. The soil conditions effect how quickly the earth shock wave loses energy. A very tightly compacted soil or bedrock will carry the energy a decent distance before friction eats away at most of the energy. A loose, sandy soil will absorb the wave relatively quickly.

Either way, the wave travels more quickly through the ground than the air. Just like an explosion in water will carry the shock wave faster through the water than it will through air.

The wave travels faster through the ground that through the air. The ground where the camera is mounted starts shaking when the ground wave gets there, before the air wave does.

>Shockwaves travels at the speed of sound…

You’ve nearly answered your own question. Shockwaves travel at the speed of sound *in the material they’re moving through*. The “speed of sound” is not one fixed speed like the speed of light, it varies hugely in different materials. So sound (and therefore shockwaves) have different speeds in different materials.

Why does it vary? Remember what sound *is* – physical particles being physically pushed away from an energy source, knocking into other particles beside them, which fly off and knock into other particles beside *them,* and so on. The closer-packed the particles are, the faster this wave of collisions-with-your-neighbor can move through the material. Because of this, the speed of sound is WAY faster in solids and liquids than it is in gases (which have very spaced out particles by comparison. The speed of sound in air is around 330 meters/sec. The speed of sound in water is 1480 m/sec. In rock, it can be 5,000 m/sec or more!

So getting back to your question: **The force that moved the camera very soon after the explosion was also the shockwave – the part of it that travelled through the ground at 5,000 m/sec and arrived much sooner** than the portion moving through air at 330 m/sec did.

Not an answer to your exact question, but related: shockwaves do not travel at the speed of sound, they travel faster.