Some days ago, the wheel in the sliding door of my house broke and thus it is creating a big noise continuously whenever I move it against the aluminium partitions. I knew and observed that friction creates this noise but I don't know the exact mechanism. So I am asking for a physical relationship. Can someone explain?
Newtonian Mechanics – How Does Friction Create Sound?
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Several things to consider: The speed of sound is much higher in water than in air. Just how much depends on a lot of factors. On http://www.waterencyclopedia.com/Re-St/Sound-Transmission-in-the-Ocean.html it says:
The state properties of water (temperature and pressure) and the degree of salinity also affect the speed of sound. The propagation of sound waves in sea water can be directly affected by suspensions of particulate matter that can scatter, absorb, or reflect the waves. Laboratory experiments demonstrate that distilled water—water from which salts and other suspended particles have been removed—provides a medium in which the speed of sound exceeds the speed of sound in ocean water. The difference in the speed of transmission is significant—speed in distilled water may be 20 to 30 times that of speeds found in ocean water.
So the type, and quality of water can make a big difference - note particularly the bit on 'scatter, absorb, or reflect', and thosw are likely frequency dependent to boot.
Next, while you can buy underwater speakers, you generally need to massage the waveform so you hear it 'properly' underwater (i.e. as if you were in air). My kids' swim team has such a rig, which is much better than trying to listen to Iron Butterfly through an uncorrected audio system in college...
- Why does friction exist in scenario 1, but doesn't exist in scenario 2?
At the microscopic level, friction is usually modeled as due to irregularities (hills and valleys) of the contacting surfaces such that when you put two surfaces together they actually make contact at very few places.
In scenario 1, when trying to move the block horizontally to move it over the surface below, the high part parts on each surface get "stuck" on one another due to the continued downward force of gravity and upward normal reaction force that keeps the surfaces in contact with one another. The applied horizontal force keeps unsticking the high points of the surfaces to maintain relative horizontal motion.
In scenario 2, however, there is no force to maintain contact between the surfaces. This allows the block, initially in contact with the surface below, to ride up the high points and separate completely from the surface below losing contact.
In short, pressure needs to be maintained between the surfaces for there to be friction. That pressure does not exist in scenario 2.
Hope this helps.
Best Answer
To understand the mechanism involved, you must know what causes friction and what happens when two rough bodies rub over each other.
When two bodies are in contact, the actual area of contact is lesser than their visible areas because the surfaces are not smooth and have irregularities (microscopic as well as macroscopic) on them (irregularities due to irregular arrangement of atoms) which we call asperities.
Friction occurs because of the bonding between the two asperities in contact. So , when you try to move an object over the other , the bonds break and new bonds form. Each time the bonds break, vibrational waves are generated (since all the atoms start vibrating) .
These vibrational waves are the ultimate source of the sound (you hear) as well the heat (you feel).
The above quote is from Feynman lectures on friction force...
Hope it helps 🙂.