From: utzoo!decvax!cca!hplabs!hao!menlo70!sytek!zehntel!teklabs!ucbcad!ARPAVAX:CSVAX:npois!houxi!ihps3!ihuxv!lew@sri-unix Newsgroups: net.physics Title: balloon explanation Article-I.D.: ihuxv.232 Posted: Mon Aug 16 08:32:45 1982 Received: Fri Aug 20 02:48:31 1982 Having received two requests for explanation of the Helium-balloon- in-the-accelerating-car effect, I'm posting this. The air in the car sloshes to the back, pushing the balloon to the front. It behaves like a bubble in a sealed bottle of water. More abstractly, in the car frame of reference, the fictitious force which pushes you into your seat adds to the force of gravity (vectorially), creating a force field slanting down and to the back. A Helium balloon, being buoyant, anti-aligns with this force field (assuming it is tethered.) Lew Mammel, Jr. - BTL Indian Hill