1. The problem statement, all variables and given/known data 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! =] 2. Relevant equations no equations 3. 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.