[Joanne Doyle]

Joanne Doyle  asked the Naked Scientists:
   
A question from a confused homeschooling mum. My youngest son, Luke (7 1/2), likes science. I like science too but get a bit lost when discussing science ideas. Luke noticed that when he had 4 marbles inside a plastic ring, if he sent another marble down an incline then it would hit the other marbles and push the end one out the ring, just like Newton's Cradle. Luke asked the following question: "If I put a thread running through the marbles and a sound wave travelled along it, would it be louder on the far side of the marble than on the near side?"

What do you think?

[yor_on]

No, I don't think so

To me it has to do with 'action and reaction' as it is called. You push something that, in its turn, pushes something else, that in its turn, etc etc. At every step of the way a little energy gets lost in friction (heat), inertia (the way objects don't want to 'pushed' out of their former state of rest, or motion, into a new configuration.)and possibly other contributions too. So energy gets lost at every step. If you think of a rope and of the sound-wave as yourself shaking that rope up and down, that wave traveling in the rope will ebb out somewhere on its way.

And the same should be the case here. If the energy didn't get lost on the way a Newtons cradle should 'tick' back and forth forever, don't you agree? And if Luke was right, even 'tick' harder and bigger the more time that went. It would be very nice if he found some way to make his idea work though. Tell him to keep thinking about it, maybe he'll spot something we miss as he grows up.

[JMLCarter]

I suspect Luke knows that sound travels down threads because of the paper cup telephone.
But it is also a kind of sound wave travelling through the marbles that transfers energy from one marble to another. It's a shock wave and cannot travel faster the speed that sound travels at in the marbles.

The marble collisions create lots of noise, which could be picked up by a detector.

But what Luke really seems to want is an amplifier, something like a megaphone that will take a small sound and make it larger. Amplification requires an energy source like some batteries.

(Focusing the sound into one direction also makes it louder - in that direction.)