I know that mechanical waves need a medium to travel through, but what happens when there is no medium and how does it it follow the first law of thermodynamics ?
In: Physics
Any vibrations would remain within the object generating them. If you make a sound on a spacecraft, it doesn’t leave. Sound waves may reflect within it. At some point, the sound dissipates and the energy becomes heat.
Waves work by one bit moving, transferring it’s energy to the next bit, and then that bit moves and repeats the cycle.
If the waves were going through an “elastic,” medium like jello (or even a sheet of steel), then when the wave encountered the vacuum there would be nowhere else to transfer the energy to, so the wave would likely bounce back and continue in the opposite direction.
If the waves were going through particles like a gas, then since the particles on the edge dont have any more particles to transfer their energy to, they would just keep moving.
Something like, say, a tuning fork makes a sound because its vibration physically pushes the air around it back and forth. That movement is what you perceive as sound.
In a vacuum, the tuning fork doesn’t have to wiggle in a medium, so it just vibrates for longer. It doesn’t go on forever since deforming the metal produces a little bit of heat, which means that eventually the vibration will slow down to a halt.
It stays in whatever object is making the sound, whatever that is. If you bang some metal in a vacuum, the waves just stay in the metal. When a wave reaches the edge of the metal, it completely turns around. This continues util that the waves are completely scrambled and incoherent, which is just another way of saying it turns into heat.
Notice how you can see your reflection in glass? That’s because light travels slower in glass than air. The bigger the difference in the speed of a wave between two things, the more the wave reflects. Same applies to sound. Some sound bounces off a wall when some goes through, and it depends on how fast the sound travels in the wall. Steel would reflect more than wood, as speed of sound in steel is about 5000 m/s and wood is 4000 m/s. Air for comparison is about 330 m/s.Since sound can’t travel in a vacuum, it all reflects.
You can visualize this as you flipping a rope up and down, and making a wave travel along it. Or better yet, go grab a rope and attach some things to the end. Attach the end of the rope to a concrete block, and when your wave hits the concrete block, it’s not going to budge, so the rope will reflect the wave back to you. The rope in your hand will start to move a lot well after you gave it a shake. Attach a heavier chain to the end, and some of the wave will go into moving the chain, some will reflect to you. Just attach more of the same rope, and the wave won’t reflect back at all, it will just continue down the rope until it dies off. You’ll feel no further shaking of the rope. Attach a light string, and some will go into moving the string, but some will bounce back to you. Attach nothing to the end of the rope, and the wave will sort of whip at the end, bounce the wave and come back, and wiggle the rope in your hands some more. That’s sound hitting a vacuum.
return to sender. if there’s nothing to collide with outside and the vibration is not enough to knock the surface atoms free then the only way to go is for everything to spring back into the material where the vibration originated.
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