Why does a human body drown if it has a density of less than 1?

Define “drowns”.

People drown frequently because they panic in the water. If they would simple relax they could float on their back or do the dog paddle in place.

I go canoe paddling and sailboat sailing nearly everyday. I can swim well and am completely comfortable in the water. I always wear a PFD (personal floatation device) not so much for me but so that I can immediately jump the water to help others who may struggle in the water. A panic stricken person would be able to grab a hold of me and not pull us both down.

People float, but you can only breathe if you are floating on your back

Not all wood floats. Humans aren’t uniform in their bodies. Some parts weigh more than others. You only drown if you inhale water. We, well, some of us, have the ability to swim and float for hours over great distances. It isn’t a matter of ‘Take human, put in water, see them sink’ like you could do for an intimate object. Even sloths swim. You invoke a principal of physics about buoyancy. I’m wondering if this is a homework problem, which we are not supposed to answer.

But does the human body weigh the same as a duck?

Human bodies are mostly water, so they are only barely lighter than water, without the added benefits of fur or feathers to increase their volume.
If you have ever tried floating on your back, you know that you have to take deep breaths constantly to keep your lungs inflated and thus keep your buoyancy up, otherwise your head submerges, and water enters your lungs.
Water in lungs = drowning.
Technically, you can drown in a puddle.

@Caravanfan Not unless they’re a witch!

The question makes no sense as asked. There’s no such thing as having a density of 1, less than 1, or greater than 1. I’m guessing you meant relative density (aka specific gravity).

@Irukandji
Water has a density of 1g/cm³.

I float so much better now that I am fatter. When I was a skinny kid I could not float easily, and there was my very overweight dad lying on the water like it was a bed. Now, I can do the same.

Staying afloat takes very little effort in calm and comfortable waters, we used to practice treading water in camp. In the ocean or fast moving waters, you might deal with waves, undertow, cold water, hot sun, swallowing salt water, and getting turned around so you swim deeper down into the water rather than up to the surface and wind up inhaling water.

The body is mostly water, so we are very near zero buoyancy. Take a deep breath and your chest expands your density decreases and you float. Blow it out and you may sink.

That’s with your head in the water. Poke your head out to breathe and you reduce your buoyancy.

I think what happens is that people have a reflex action to try and breathe, even when it is not safe to do so. Therefore, they end up inhaling a lot of water. I believe that is what causes drowning.

@ragingloli I know that. But 1g/cm³ is not the same as a bare 1. The lack of units was exactly what I was pointing out. And relative density is what determines whether or not something floats.