Why Does the Van der Waals Equation Adjust Pressure Up and Volume Down?

[Qube]

Homework Statement

Why does the Van der Waals equation adjust pressure up and volume down?

Homework Equations

(P + a)(V-b) = nRT (overly simplified, but it gets the point across).

The Attempt at a Solution

Okay. Volume refers to total free volume in a container in the context of the gas law. I can understand why volume is adjusted down relative to the ideal gas law equation (PV = nRT). Gases aren't point masses and actually occupy some volume.

However, why is pressure adjusted up? Gas molecules may attract and participate in inelastic collisions. This would seem to decrease the actual pressure. Why then would pressure need to be adjusted up?

 

[DrClaude]

Gas molecules may attract and participate in inelastic collisions. This would seem to decrease the actual pressure.

If you were to measure the pressure of a gas with $a = 0$, and then, with everything else the same, with $a > 0$, how would the measurements differ?