Why does a cracked whip make a sound? One portion of the whip moves faster than the speed of sound, creating a sonic boom.
A sonic boom is created by anything that goes faster than the speed of sound, which is 343 m/s in air. Air behaves in the same way as a fluid, so it moves in a very similar way to water. You can have waves in the air in the same way as you can have waves in the ocean. Ocean waves are often caused by the wind, and they form because they are transferring energy through the water. It looks like the water in the sea is moving along causing the wave, but the individual molecules don’t move very far. The energy makes the molecules move more, which raises the level of the sea, but that energy is very quickly passed onto the next molecules and the next, carrying the wave with it. The wave is the motion of energy and not of water. The same thing happens in the air. When we make a loud sound, the energy we produce with our lungs is transferred to the air molecules closest to us, making them move more. These molecules collide with the molecules closest to them, passing the energy on to the next molecules and so on and so on, making a sound wave. As the molecules collide with each other, some of the energy is lost as heat and the sound wave slowly dissipates. The molecules pass on the energy at a speed of 343 m/s.
A sonic boom happens when something travels faster than the speed of sound and produces more pressure than can dissipate. If you make a sound, the energy from the sound travels in every direction. With your voice, most of the energy goes forward because your head blocks it, but the sound wave goes in all directions. It is the same as when you throw a rock in a pond. Planes are a common source of sonic booms, but the sonic boom isn’t caused by the sound from the plane, it is caused by a pressure wave. If you push your hand through a swimming pool, you are pushing the water and creating a pressure wave. Planes produce pressure waves as they fly through the sky and pressure waves travel through the air at the speed of sound. When a slow plane is flying through the sky, the pressure waves spread out all around the plane and dissipate. As the plane gets faster, the pressure waves in front of it start to build up because the plane has pushed more air in front of it before the air that was already there can dissipate. This produces higher and higher pressure. Once the plane breaks the speed of sound, it is travelling faster than the waves of pressure and the waves in front of the plane become compressed, releasing all of the energy at once as a shock wave. This is the sonic boom. They were first studied by an Austrian physicist called Ernst Mach, hence the reason we say Mach 1 for one times the speed of sound.
Ok. Back to the original question. Why does a cracked whip make a sound? It is because the whip makes a sonic boom, but, surprisingly, not the tip of the whip. Alain Goriely from the University of Arizona and Tyler McMillen, a graduate student of mathematics, analyzed the whip crack. They discovered that when the crack of the whip is heard, the tip of the whip is already travelling at twice the speed of sound. If the tip of the whip was the cause of the crack, then the sound should have been heard earlier because a sonic boom is caused at the point where something exceeds the speed of sound.
When the whip is cracked, the handle is flicked in a twisting motion. This produces a loop in the material of the whip that starts by the handle and travels down the length of the whip before ending at the tip and giving it the flip that we know from whips. The loop starts off fairly slow because it only has the energy imparted by the flick of the wrist and the arm. However, because whips are thick at the handle and slowly taper to the very thin tip, the speed of the loop increases as it travels along the whip. By the time the loop reaches the tip, it can be moving at over 30 times its original speed. This means that, at some point along the length of the whip, the loop reaches the speed of sound and produces the sonic boom. The loop then continues to accelerate until it flicks the tip of the whip at twice the speed of sound. The sound comes before the whip reaches the end but, because of the distance to our ears and the speed with which our brain processes the sound, it sounds like the crack is coming from the tip. And this is what I learned today.
sources
https://science.howstuffworks.com/question73.htm
https://www.af.mil/About-Us/Fact-Sheets/Display/Article/104540/sonic-boom/
https://oceanservice.noaa.gov/facts/wavesinocean.html
https://oceanexplorer.noaa.gov/facts/waves.html
https://www.phys.uconn.edu/~gibson/Notes/Section6_4/Sec6_4.htm
https://wonderopolis.org/wonder/what-is-a-sonic-boom
https://www.scientificamerican.com/article/true-cause-of-whips-crack/