Sunday, February 8, 2009

Why do soap bubbles float in the air? A follow-up

My dear mother-in-law (the good kind) asked in a comment why soap bubbles seem to float in the air, all the while maintaining their (approximate) original shapes.

She also asked if my son actually understands what I post. I (usually) go into more detail here than I do with him. He is, after all, only six years old. I haven't quizzed him. This is more to get him excited about asking "why?" in a more permanent way than the "why? phase" all 3-5 year-olds go through.

So, the soap bubble maintains its shape because there's not enough force on it from outside (or inside) influences to overcome the cohesive forces that keep the water together. Once any one spot is broken, the entire bubble will collapse because surface tension is no longer competing with itself.

The bubble floats as long as there is some force (usually a breeze, a child's breath, etc.) pushing it up even slightly. Whenever gravity times the bubble's mass is the dominant force, the bubble falls because the buoyancy force being applied by the atmosphere is too weak compared with that downward force.

1 comment:

deborah said...

Thanks for the answer. Just wanted you to know that when I talked with Garion yesterday he did not want another snow day because he was going to miss science day with his dad....=)! Sometning he really looks forward to.