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I understand this all about density.
Now the downward force on a single bubble = gm where g is the acceleration due to gravity, and the drag force due to air resistance is proportional to its surface area a. So the terminal speed will be a function of m/a.
Quote from: gazza711 on 23/05/2015 19:33:45I understand this all about density.If you blow 2 bubbles of similar size and watch the rate of fall, when you then push them together they have the same weight and volume so should sink at the same rate. The only way they can sink faster is if they have gained weight or lost volume. They clearly can't gain weight.How have you performed this experiment? Have you eliminated air currents from the area?. How do you get the bubble to combine?
Air pressure compression between bubles?
Alan - I love your take on this; I had assumed, when he said "there are many" that he meant many individual bubbles, rather than a linked raft, which is the interpretation you have used, isn't it? Many individual bubbles should all behave individually and independently like a single bubble, assuming that they don't interfere with their neighbours.
I simply blew a bout 10 bubbles.there were three larger ones left.they fell slowly from 6ft and combined at 5ft and accelerated very fast to the ground.There were no drafts or any other forces involved.
Quote from: gazza711 on 25/05/2015 08:33:11I simply blew a bout 10 bubbles.there were three larger ones left.they fell slowly from 6ft and combined at 5ft and accelerated very fast to the ground.There were no drafts or any other forces involved.When the bubbles combine they don't gain weight ie weight of bubble 1 + bubble 2 = weight of combined bubble. Also they don't lose air volume, none has seeped out. So it's not a question of density.Alan is suggesting it is air resistance. Try these 2 experiments with your daughter.Take 2 equal sized pieces of card, drop them one flat side down the other on edge. Clearly the one on edge falls faster, but their weight and volume are the same.Next take 2 equal sized downy feathers like you get in pillows. Drop both together they will fall slowly. Now crush one so no fluffy bits and then drop them. Crushed one falls faster, but they both still have same weight. In both cases air resistance causes the different fall rate.It's the same with the bubbles, the air resistance has changed.Will be interested to see the video. Edit: forgot to say, look out for what David mentioned, smaller bubble hanging below larger. These should fall faster because of lower resisting area, but increased weight of combined bubble.Also, air currents in bathroom. Over the bath warm moist air will rise, when it reaches the ceiling it will move outwards and fall over the cooler area outside the bath edges. So as long as the bubbles are within the bath perimeter you could view them as being in the same current, but don't let them go outside otherwise they will fall even faster.This is great, real science, I'm almost tempted to get in the bath with my bubble kit!