16. "How does a gun silencer work?"

I'm not sure that this has much to do with "Physics for Engineers II" but that's won't stop me from attempting an answer! I believe the answer to your question is quite simple really. When a bullet is fired from a gun, gunpowder is ignited in the shell case that produces gas at a rapid rate creating a high pressure pulse behind the bullet. The force of the gas does work on the bullet and by the work-energy theorem (semester 1) leads to an increase in the kinetic energy of the bullet. When the bullet emerges from the end of the barrel, the pressure of the gas bhind the bullet is still very great - ~200 atmospheres - and so its rather like a cork "popping" from a bottle of champagne. As you know that creates a loud noise.

To reduce the noise, a silencer is attached to the end of the barrel. A silencer is simply a large volume, some 20-30 times greater than the volume of the barrel. What that means is that when the bullet exits the barrel, the excess gas now expands into the large volume before existing the silencer. As a result, the silencer reduces the pressure of the exiting gas and the sound is much reduced. The greater the volume of the silencer, the greater the reduction in pressure and sound.

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