potto said:
TL;DR Summary: question about gravity, weight and added mass feeling.
The question: do we really heavier in water? If it is not the case, what makes us think that we are heavier? Is it a physical phenomenon that it actually demand additional force?
There must be a name for this where cause and effect are somewhat scrambled in the brain, in this case the feeling and conclusion that being immersed in water makes you ' heavier'.
I have even felt it not so much as when coming out of a bath, but fully noticable when raising an arm out of the water when lying in the bath. The arm really does feel as if it has become way much heavier.
It is a phycological mind trick.
Bear in mind that bouyancy is a wonderful thing. Astronauts train under water in their space walk suits giving them the impression towards weightlessness. Except for the difference of viscosity of actual space ( which is it doesn't have any ) to that of water, the experience would be as if the astronaut is 'floating' in space. The forces that his/hers limbs would have to overcome for movement would be that of the flexing of the suit and moving the limb, or body, through a viscous fluid ( the water ). Since the gravitational attraction of the astronaut towards the centre of the earth is compensated by their bouyancy, I would imagine that the experience is somewhat akin to doing a spacewalk along side their craft in freefall orbit around the earth.
Enough about astronauts.
How could you, or anyone else, come to the conclusion that being in the water makes you heavier?
Normally, out of the water, when we move our body, or any part of it such as limbs, we are accustomed to having to exert a certain force, and think not to much about it.
When in the water, we can move our bodies, and our limbs, and not think to much about it also, as long as the movements are not to drastic.
That is until when when we try to run forward, or much an extended arm quickly in an arc, as two examples. In these instances, we quickly do realize the there is the water all around us, and being the thinking creatures that we are, the density and viscosity of the water must play a part in hindering the velocities and accelerations that we are so accustomed to when immersed in air ( ie where we normally live our lives ).
But try to lift an arm slowly ( probably not as noticeable if movements are quickly done ) out of the water. Or try to lift your whole self out of the swimming pool. The feeling does come about that the arm, or your body, has somehow become heavier when it leaves the water.
Your brain using its own logic does come to the erroneous conclusion that we have grown heavier. We don't have a bouyancy centre within the brain that tells us " OK, extra muscle force needed, arm is coming out of the water ". The extra force is the difference of the bouyant arm ( not much force to move upwards ) to that of weight of the arm in air ( a greater force ) . Since this happens say in less than a second ( a step function if you are into systems control ) we do think about it - ie we have to exert more muscle contraction to lift the arm higher from the water into the air. The brain, from somewhere deep within its 'erroneous centre' scrambles for a cause, and concludes that the arm has become heavier.
PS. Besides astronaut training , some physiological therapies do involve exercises while being immersed within water.