Show that inside a soap bubble, there must be a pressure [itex]\Delta P[/itex] in excess of that outside equal to [itex]\Delta P = 4 \gamma /r[/itex] where r is the radius of the buble and [itex]\gamma[/itex] is the surface tension. My first question is: The pressure on what!? Are they referring to the pressure on the bubble itself or on the surface enclosed by the bubble? I already "solved" the problem, but I did it without knowing how and why (I just plugged in what I thought would work and it worked!). For one thing, they seem to be referring to the pressure on the surface enclosed by the bubble. How is the bubble applying pressure here? It seems the surface tension is responsible for keeping the bubble from bursting, but I don't really know why.