All sound waves need a(n) to travel through. 4. and can be used to describe a sound wave.

Material medium

compressions and rarefactions

Explanation:

A sound wave is an example of a mechanical wave. All mechanical waves require a material medium for propagation. The medium for the propagation of sound is air. This is the reason why, if you cover your mouth, it will be difficult for another person to hear whatever you are saying.

Sound is also a longitudinal wave. Longitudinal waves are described in terms of compressions and rarefactions. Compressions refer to areas where air molecules crowd together while rarefactions refer to areas where the air molecules spread out.

cant without the picture! ?

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.