Question: A spherical rubber balloon inflated with air is held stationary, with its opening, on the west side, pinched shut.
(a) Describe the forces exerted by the air inside and outside the balloon on sections of the rubber.
(b) After the balloon is released, it takes off toward the east, gaining speed rapidly. Explain this motion in terms of the forces now acting on the rubber.
(c) Account for the motion of a skyrocket taking off from its launch pad.
(a) The air inside pushes outward on each patch of rubber, exerting a force perpendicular to that section of area. The air outside pushes perpendicularly inward, but not quite so strongly.
(b) As the balloon takes off, all of the sections of rubber feel essentially the same outward forces as before, but the now-open hole at the opening on the west side feels no force – except for a small amount of drag to the west from the escaping air. The vector sum of the forces on the rubber is to the east.The small-mass balloon moves east with a large acceleration.
(c) Hot combustion products in the combustion chamber push outward on all the walls of the chamber, but there is nothing for them to push on at the open rocket nozzle. The net force exerted by the gases on the chamber is up if the nozzle is pointing down. This force is larger than the gravitational force on the rocket body, and makes it accelerate upward.
So I have a few questions.
(a) Why is the force outside less strong? Does the author mean it's less strong temporarily until the balloon is inflated and is in equilibrium? What provides the force from the inside -- compressed air molecules? What about the outside force? Where does the tension of the ball come into play, if at all?
(b) What pushes against what here? (Drag hasn't really been explained by the author). Is it the air inside the balloon pushing westward on the air outside and the air outside pushing back on the air inside the balloon which transfers all the way to the front of the balloon, pushing it eastwards?
(c) The chamber walls are able to provide equal force in the opposite direction of the force of combustion products keeping the walls in equilibrium. But what happens at the hole? Combustion products push against the air below and the air pushes on the combustion products which transfers to the top?