Energy, Sound and a Vacuum?

Question

I'm sure you've all seen those executive toys with the metal balls on strings where you pull one back, let it go and it hits the others making the one on the other end swing out and back in.

Take two of these, place one on a table and one next to it in a vacuum then pull the same ball on both toys back the same distance and release them. It's safe to say that the one in the vacuum would carry on running longer than the one outside the vacuum as there's less resistance in the vacuum, however that is an assumption on my part so correct me if I am wrong.

One of the toys will be making a "Click" every time the balls come in to contact and the one in the vacuum wont as there's no way for the acoustic wave to propagate.

If there's no way for the sound to propagate does it actually make a sound in the first place ?

If it doesn't would the one in the vacuum run longer as there's no energy wasted on sound and if it does where does the wave go to when it can't propagate ?

Answer 1

As the two balls come into contact, since there is no air then no sound can propagate away from the balls. However there WILL be sound that travels through the balls themselves. This is similar to the shuttle docking with a space station. The passengers inside the shuttle and space station will hear a sound but if a microphone was placed just outside the craft then nothing will be heard. Does that make sense?

Anyway, there will be sound but it can only travel within the balls.  Sound can travel through solids, liquid and gases, but it cannot travel through a vacuum.

Hope that helps :-)

Answer 2

If there's no medium for the sound wave to propagate, there will be no sound. The pressure wave will reflect back on itself and keep the toy going.

And, like you yourself said, without the air resistance, the toy will run longer.

Answer 3

This would produce no audible sound, but [nearly all] the energy used to produce sound would still be lost.

When two balls hit each other, they deform slightly (since there is no such thing as a perfectly elastic collision). This deformation occurs very rapidly, causing the balls to vibrate a bit, which in turn causes the air around them to vibrate, producing what you hear as a "click".

In a vacuum, the two balls will still deform for an instant, and they will vibrate, just as they would in air. The only difference here is that there is no gas around the ball for it to cause to vibrate, so more of this vibrational energy remains in the ball, rather than being converted to sound waves.

A lot of energy is lost to heat though. If you take a nail and bend it 90 degrees, you'll notice that it gets very hot around the bend. The same thing happens when the balls deform, and this heat is lost in air or vacuum.

As far as where the wave goes - it never even existed. Sound is a "matter" wave, unlike light, radio waves, etc. If there is no matter (gas) to react to a moving object, no sound wave is ever created.

And yes, your assumption about the toy in the vacuum running longer is correct; as there is no fluid damping the motion. When the toy in the vacuum comes to rest, almost all of the initial energy would have been lost to heat.