Why Can Mechanical Waves Propagate Through a Medium?

[eureka_beyond]

Homework Statement

This is a general concept question. The problem is, I don't seem to understand what the question is saying and why. Here's the question:

A mechanical wave can propagate through a medium if

a) there are attraction/ repulsion between the medium particles
b) the particles are able to vibrate about their equilibrium position
c) the particles have a net displacement when they are subjected to disturbance

Only a) and b) are correct, but why?

Thanks! =]

Homework Equations

no equations

The Attempt at a Solution

To be specific, I don't understand how there are attraction or repulsion between particles. Then I don't understand how is the vibration of particles important when they are in their equilibrium position. Thirdly, why is c) incorrect. I'm totally confused.

 

[hikaru1221]

1 - Imagine that the particles are like masses and their interaction is like springs attached to them (this is a simple model). If you excite the system by kicking the 1st mass for example, its motion will excite the 2nd mass to move, which will make the 3rd one to move and so on.
If there is no interaction, i.e. attraction or repulsion, it will be an ideal gas. But wave propagating through gas is another story. I guess the question doesn't apply to gas (though wave through gas is mechanical wave), because the answers focus on the vibration of each particle.
2 - They actually always vibrate. Answer (b) means the particles can vibrate, not be in free motion like gas.
3 - I guess answer (c) means the wave makes the equilibrium position change. You know, when vibrating, particles only vibrates about its equilibrium position, so after one period or when the vibration stops, it comes back to the same position, which means no net displacement. But it's just my speculation.

 

[HallsofIvy]

No, the correct answer is NOT (a) and (b), it is (b) and (c). Water waves, for example, occur because, after disturbance, each water "particle" is returned to its position by gravity. There is no "attraction/ repulsion between the medium particles".

 

[eureka_beyond]

Ah~ I better correct the answer in my exercise book. Now I get it why a) is incorrect and why b) is correct. But I'm still having few concerns with c). After disturbance, the particles return to their original position, so there's no net displacement. c) saids there is. So i guess c) must be discribing the situation where during disturbance, there are some particles which does have displacement, and so there is net displacement. It's talking about all the particles as a whole rather than any particular one. Am I right? Or am I just making it more complicated?