Question

which phrase describes air density?
○ increases as altitude increases
○ equals mass divided by volume
○ pushes molecules in one direction
○ exerts less pressure as it increases

Explanation:

Step1: Analyze each option

• Option 1: Air density decreases as altitude increases. So this option is wrong.
• Option 2: Density is defined as mass divided by volume ($

ho=\frac{m}{V}$). This is the correct formula for density, including air density.

• Option 3: This description is more related to force or pressure - like concepts (e.g. net force on molecules) rather than density.
• Option 4: As air density increases, the number of air molecules per unit volume increases. According to the ideal gas law ($PV = nRT$), if we consider a fixed - volume container (approximate for a local air parcel in some cases), more molecules (higher density, since $n=\frac{m}{M}$, $M$ is molar mass) mean higher pressure. So this option is wrong.

Answer:

equals mass divided by volume