# Calculate size of bag needed to stay buoyant [closed]

First of all, I know nothing about physics so please be gentle on me with the technical speak :-)

I've thought of a game that my son and I can play in the swimming pool, he is 11 and weighs around 55kg's. I weight around 96kg's.

One of the requirements for the game involves creating a bag that can keep us buoyant. While a bag that can support my weight would also suit a small child, I would like to create a smaller bag for children.

The idea here is that the bag will be in front of us and we can put our arms over it and just float.

The bag should be as small as possible so that it doesn't get in the way too much, so I am thinking of the bag being roughly the size of an A4 sheet of paper.

My questions are:

• Is a bag this size (A4) capable of holding up an adult of say 120kg's?
• What should/could the bag be filled with to assist in the buoyancy, helium etc?
• Is this even possible, and if not, what would the minimum size of the bag be to support a 120kg human?
• How would the calculations change if I was to use it at sea with the buoyancy being provided by sea water?

I've tried to calculate the buoyancy myself, but there are just too many unknowns for me, how do I calculate the volume, how do I know how much will be submerged, do I need to add the weight of the human to the calculation etc?

A step by step guide on how to calculate it would be massively appreciated if its not too difficult.

Update 1: I've come across this site http://www.wikihow.com/Calculate-Buoyancy which gives detailed steps on how to calculate buoyancy. Having performs these steps with my weight (less the weight of my head +- 5kg's because that will be outside the water), I get 905.978 newtons required. I've then calculated my total weight (including weight of head) and got a total buoyancy force of 941.76 newtons. I.E. I'll sink, which is obvious and that's why we need to wade in water. So my thinking now is that if I were to create a bag for the difference of +- 36 newtons, then I'd float. Is that thinking correct?

## closed as off-topic by David Z♦Aug 7 '16 at 11:03

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question appears to be about engineering, which is the application of scientific knowledge to construct a solution to solve a specific problem. As such, it is off topic for this site, which deals with the science, whether theoretical or experimental, of how the natural world works. For more information, see this meta post." – David Z
If this question can be reworded to fit the rules in the help center, please edit the question.

• Hi Steve ! This question does not fit within the site guidelines for asking questions. I hope you (re) read it and edit the question. – Gaurav Aug 7 '16 at 10:20
• Apart from what Gaurav said, this question cannot be answered because you haven't given us information regarding the density of the bag material. You need around 12L bag to keep a 120 man afloat assuming that the bag is massless, you would probably need around 15L . Use the formula , "weight to float = 9.8 * volume" where volume is in L. Don't forget to take account of the weight of the bag itself. – Yashas Aug 7 '16 at 10:37
• Hi Gaurav, As a software developer, I'm used to stackoverflow, which is a similar concept. From my side, I've tried to explain the problem, and I've given very specific questions via the bullet points. I'm not sure where I have missed the guidelines, please let me know and I'll gladly change the question. Thanks, – Steve Aug 7 '16 at 10:46
• Hi Yashas, I am thinking of a lightweight foil bag (I have no idea how to describe it), but a bag made of something like a bag of chips (but obviously a little stronger so that it wouldn't puncture) Thanks for the basic info of 12L bag. I'll look into that. – Steve Aug 7 '16 at 10:57