Air Pressure vs. Air Pressure

To inflate a balloon, the air pressure being blown into the balloon must overcome or be greater than the static air pressure of the environment outside the balloon.

Air Pressure vs. Surface Tension

To inflate a balloon, the air pressure being blown into the balloon must overcome or be greater than the balloon's surface tension, or stretchiness.

Cumulative Pressure

A balloon inflates whenever the sum of all pressures inside the balloon exceeds the sum of all pressures outside the balloon. Therefore, to inflate the balloon, the pressure inside from adding air must overcome both the outside air pressure and the balloon's surface tension at the same time.

Inflation by Withdrawing Air

A reduction in outside air pressure can inflate a balloon if the inside air pressure remains unchanged. For example, a sealed balloon that is placed in a vacuum chamber or taken high into the atmosphere in an airplane will inflate.

Equilibrium

A balloon is in equilibrium when the total net forces on the inside and outside of the balloon are equal and opposite. When a balloon is in equilibrium, it will not inflate or deflate, but remain the same size.

Source:

Physics I: Classical Mechanics

The Physics Department: Mechanics