Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 14 - Think and Explain - Page 279-281: 78

Answer

The balloon's stretched material, in trying to return to its original size, provides an inward pressure. The gas pressure inside must overcome this pressure plus the outside air pressure. (Additional proof: stick a pin into the balloon. If the inside and outside pressures were equal, there would be no tendency for air to flow to equalize pressures, and it would be an unremarkable event.)
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