Will a door still float if you open it?


There’s a thought-provoking allegory in a novel I’ve just been reading, A. M. Homes’s deliciously bizarre ‘This Book Will Save Your Life’. It goes like this:

A flood sweeps through a man’s house, washing it away. He is carried off in the torrent but a door floats past and he grabs hold of it and pulls himself aboard.

The deluge takes him along a road, now a fast-flowing river. Soon, however, all he can think of is what is behind that door. Eventually, he can bear it no longer and he turns the knob, opens the door and falls in and drowns.

I like that. But now, like that man on the door, the thought of what’s behind it is plaguing me.

If you were to open a door floating on water, you wouldn’t need to fall through it to drown, would you? Wouldn’t it stop being buoyant as soon as you opened it? Or could you save yourself by quickly flipping the door right over on to the surface of the water?

Could Archimedes save my life? If you have the answer, please email me here. 


‘Mr Archimedes he say, in practice, that the volume of the door below the surface, when horizontal, supports the man, so it takes the same volume of the door on edge to support the man – assuming he has good balance.

If one was being pedantic, one could ask how the man was able to open the door whilst sitting on it unless the frame were large enough to provide a foothold as well and enough buoyancy to support him, in which case he could do what he liked with the door.

Alternatively, if he was a great surfer, and there was no door frame, he might be able to stand on the door, flip it onto its edge and rail board it with a tiny bit of freeboard until he flipped it over, so opening the door but then he could remain standing on the now open door.’Southampton Simpleton.

‘The first question must concern the consruction of the door. If it can be opened, it must still be in a frame, so how large is the frame, and how much buoyancy is it providing? Also, consider the door itself. Is it hollow or filled with an absorbent material e.g. a fire door? if so it may be filling with water and may achieve neutral or negative buoyancy.

When the door is opened, there is no change to its potential buoyancy, but there will probably be a change to the amount of the body immersed, which will alter the buoyancy.If the door is opened to below the surface of the water, the buoyancy will increase, making the surrounding frame float higher. If the door opens upwards, the initial change will give less buoyancy, but eventually an equilibrium will be reached which will leave the door and frame floating in the same manner as before, but with a different orientation.

Consider a sealed boat hull. It will float whichever way up it is, unless water penetrates the hull. The door will behave in a similar manner, so will still float until the structure is altered to make it denser than water.’Jon Martin

‘Of course your door will still float, whichever way you choose to open it.

Should the door swing ‘up’ relative to its frame, then simply some more frame – and maybe a bit of door – will immerse itself until Archimedes’ Principle is satisfied again. Similarly, should your chosen door swing ‘down’, immersing more of itself in the ogin, then the whole kit ‘n caboodle will rise up a bit, to Mr Archimedes’ satisfaction. Much the same thing if said door is on edge.

This problem is probably best offered to the International Moth sailors, who will likely fit foils and get the old door planing round an Olympic Triangle at indecent speeds, and/or the Amateur Yacht Research Society nuts, who have already tried this out at Weymouth Speed Week many times and are now sailing virtual doors Downwind Faster Than The Wind all over the internet, to the consternation of bulletin boards everywhere.

Either way, you will probably still need to hike your door to windward, except in very light airs…..’Bilbo