Final answer:
The force keeping an inflated balloon open is the force determined by the gas molecules colliding with the balloon's inner walls, which creates pressure inside the balloon.
Step-by-step explanation:
The force holding the walls of an inflated balloon open is the force of the molecules hitting the inside of the walls of the balloon. Each time the gas particles strike the walls of the balloon, they apply a force. An increase in the number of gas particles or their velocity, due to temperature for instance, will result in more collisions, applying a greater force, thereby increasing the pressure on the inside walls. This pressure is what keeps the balloon inflated. The attractions between gas molecules can decrease this force under certain conditions, but typically, it's the collisions that maintain inflation. It is important to understand that the elastic collisions between the gas molecules and the balloon walls, according to the ideal gas law, also relate to molecular kinetic energy and temperature.