Let's say you had 3 identical air tight bottles, each has a one way valve. What happens if you?
Let's say you had 3 identical air tight bottles, each bottle has an air tight one way valve. If bottle # 1 has 3/4 of it's air vacuumed out, bottle 2 leave the valve open so air pressure is the same inside and out, bottle 3 you pump in some helium. Do all 3 bottles weigh the same when put on a scale?