A boy is riding a sled down a snow covered hill. A girl then jumps onto the sled as it is riding down the hill. Briefly explain what will happen to the (i) coefficient of friction when she jumps on and (ii) the force of friction when she jumps on.

Sounds propagates through some kind of medium. usually it's air, but it goes just as well through solid objects like walls, windows etc. a soundwave will compress the (air)molecules in front of it, and if this increased density wave hits your ears, you register it as sound. solid objects like walls are a lot denser than air, this means that they will damp a wave, but also carry it easier.