h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Does a balloon weight more/less/the same after it has been inflated compared to when it was deflated?

A balloon weighs more after it has been inflated due to the weight of the air inside it. Unless you use something like helium, which is lighter than air. But even then I think the balloon will (technically) weigh more, or at least have more mass (if you want to be pedantic) because of the gas inside it.

A lot of people don't seem to appreciate that air has weight/mass. Atmospheric pressure is 15 p.s.i. (pounds per square inch), which may sound a lot, but is balanced by the internal pressure of our bodies.

It would also depend on the altitude the ballon would reach on being inflated. The further it would get from the centre of gravity ie the earth's core the less the gas used to inflate the ballon and the fibre that is the ballon would weigh.

Also CD was slightly incorrect about the effect of filling with Helium. The helium would expel the Air consisting of Heavier elements O, H, C, Ar etc thus making a Helium filled balloon lighter. If it is just being filled with hot air though the weight remains the same but the mass would increase as the air particles would agitate more at a greater velocity.