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Do you feel macho ?

OK you might ask yourselves why I used the phrase “Do you feel macho ?” in the title of a blog on science matters but things will become more clear soon. 

This is a post about the “Cartesian diver”, some years ago I found on the web an easy method using a ketchup packet. Which I tried using sauce packets from the local fish and chip shop. 


I have found an alternative which is even more cheap. Get cut out a small bit of bubble wrap, then add paperclips to it until it only just floats. Then put it in a empty plastic water or coke bottle. 

Next fill the bottle with water and screw on the cap, now squeeze the bottle until the bubble wrap sinks. Now let go and it floats. 

If you want you can put a series of different puts of bubble wrap in there and then the harder you squeeze the more will sink. If you are a bunch of competitive teenaged boys then you could always use a 3L lemonade bottle and then see who can squeeze the hardest on the bottle and make the most bits of bubble wrap sink. Thus the Cartesian diver can be used as a “try your strength machine”. This idea is the origin of the “Do you feel macho ?” title. 

Do not squeeze so hard that you strain or break your hands or make the bottle burst ! 

Disclaimer, while Dr Mark Foreman takes all reasonable care to create safe and enjoyable experiments I accept no responsibility for any death, injury, disease, property damage, prosecution, criminal court case, tort, civil court case or other adverse outcome resulting from you trying out an experiment of mine or using any information I provide. All experiments are done at your own risk. If you are not an adult you must consult your parents and/or science teacher before trying an experiment. If you are doubt about your ability to do the experiment in a safe manner then do not attempt it. Also I accept no responsibility if your mum/dad/teacher/policeman/(insert some other authority figure) tells you off for doing the experiment. Also I am not responsible for your moral, social and spiritual wellbeing. Also if you hate the experiment and do not enjoy it or are left with an intense feeling of self-disgust then again I am not responsible. If you read this disclaimer or anything else I write and have some adverse reaction to my words such as going insane, laughing yourself to death, pushing your blood pressure through the roof or some other effect then again I accept no responsibility. 

Science time 

The volume of a gas can be calculated using PV = nRT which is the ideal gas equation. 

The downwards force on the bubble wrap plus paper clip due to gravity is given by F = Mg 

The upwards force on the bubble due to buoyancy is given by F = V density g 

When Mg > V r g then the bubble wrap sinks but when Mg < V r

I have done a few thing to make the maths more simple, firstly I have treated the density of the gas as being zero. Real gases have densities but these are low when compared with that of water. 

Water has a desnity of 1000 g per litre, while air has a density of about one gram per litre. If you want a super dense gas then consider radon, which has a density of 9.25 grams per litre at 20 oC and 1 bar. 

If you do not want to treat the gas as being an ideal gas, then feel free to do the more complex maths. But I do not think that you would get a great advantage in this case.


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