When we hear the term “shockwave,” it often evokes dramatic images: the powerful blast from an explosion or the aftermath of a supersonic jet. In everyday language, we even use “shockwaves” metaphorically to describe major events that send ripples through society, like an election or a breakthrough discovery.
But what exactly are shockwaves and how do they come to exist? In this article, we’ll delve into a mini physics tuition crash course on shockwaves, uncovering their unique properties and the fascinating mechanisms behind their formation.
What Is A Shock Wave?
A shock wave is a type of disturbance that moves through a medium, causing abrupt changes in pressure, temperature, and density along a very thin wavefront. Unlike regular waves, shock waves form when an object moves at supersonic speeds—meaning faster than the speed of sound in that medium. An example is when a bullet travels through the air. Scientists often use techniques like Schlieren imaging to visualise these shock waves and better understand their structure.
To fully grasp how shock waves form, it’s helpful to understand the concept of the Mach number. This term represents the ratio between the speed of an object moving through a medium and the speed of sound within that same medium. When an object reaches Mach 1, it’s travelling at the speed of sound, a speed also known as “transonic.”
Imagine a boat speeding through the water. If it moves faster than the waves on the water’s surface, it creates a visible wake as the waves shift out of its path. This effect is similar to what occurs when an object moves through air at various speeds.
When an object moves slowly, with a Mach number less than one, the waves it generates spread symmetrically in all directions around it. However, as its speed increases, the waves in front begin to compress, forming a higher frequency, while the waves behind spread out. At the exact speed of sound, or Mach 1, these waves crowd together, overlapping at the front of the object.
Once the object surpasses the speed of sound, it creates a shock wave. The waves are now unable to move out of the way fast enough, forming a distinctive cone-shaped pattern, often called a Mach cone, which trails behind the object as it moves forward.
Shock Waves Around Us
There are a lot of things that can produce shock waves, most notably the modern fighter aircraft many militaries possess today as they can travel at speeds beyond Mach 1. When a supersonic jet breaks the sound barrier, a visible vapour cone often forms around it due to the sudden drop in temperature and pressure caused by shock waves. When these shock waves reach the ground, they create the loud “sonic boom” we associate with fast-moving jets. As mentioned, most high-velocity bullets travelling at supersonic speeds also generate shock waves. This is what creates the distinct “crack” sound as a bullet passes you by, caused by the air it displaces at high velocity.
Even the familiar crack of a whip results from shock waves. When the tip of the whip moves faster than the speed of sound, it produces weak shock waves that give rise to that distinctively sharp crack. Shock waves can also form in supersonic fluid flows. While we typically think of shock waves around objects moving at high speeds, they also arise when fluids, such as gases, flow through a pipe at supersonic speeds. When the pipe’s dimensions or shape change suddenly, the flow encounters an obstacle, creating shock waves in response to the abrupt shift in conditions.
In short, shock waves are more than just a feature of high-speed objects; they also appear in various natural and industrial settings, making them a fascinating and versatile phenomenon in physics.
Conclusion
Shock waves are powerful and intriguing phenomena that occur when objects or fluids exceed the speed of sound in a given medium. From the sonic boom of a supersonic jet to the crack of a whip, shock waves are all around us, impacting everyday experiences and specialised applications alike. By understanding the mechanics behind shock waves—how they form, interact with different media, and affect surrounding pressures and temperatures—we gain insight into both the dramatic and subtle effects they produce.
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