State archimedes principle why it is easier to swim in sea water than in river water
Answers
Answer:
Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.
It is easier to swim in sea water as it has a higher density due to the presence of salt. This results in a higher value of buoyant force according to Archimedes principle.
Answer:
Its easier for the swimmer to swim (that is float) in sea water than fresh water because density of sea water is more than fresh water.
Below is an explanation without going into detailed physics formulas of density and buoyancy. I am not sure how much you know about buoyancy, density and how dissolved mass affects density of fluids so I will try my best to keep the explanation as simple as possible.
Take a glass put a fresh egg in it and fill it with water. The egg should stay at the bottom of the glass (if the egg floats then you have a bad/ rotten egg and need another one!). Take some salt and put it in this glass stir it and see if the egg floats. If it does not add some more salt and stir it until it dissolves . Keep repeating this and eventually the egg will float! You may need quite a bit of salt before the egg floats but it will float eventually. I have done this many times with kids and they are always amazed.
Now why did the egg float when salt was added to the water but not before when it was in fresh water? Well by adding salt to the water you increased the density of the water which in turn increased the buoyancy of the egg. Density tells us how compact a substance is and buoyancy is the ability of something to float in a fluid (that is any fluid not just water).
Fresh water has density of about 1000kg/m^3 while sea water density ranges from 1020kg/m^3 to 1030kg/m^3, depending on temperature of the sea water and how much salt is dissolved in water at that particular place. So because sea water is more dense (because it has salt and other substances dissolved in it) the swimmer finds it easier to float/ swim in salt water than fresh water.
It is also why oil floats on top of water and the reason why people cannot sink in the Dead Sea! The Dead Sea is so sooo salty that the density of water is 1240kg/m^3 so you can't sink in even if you tried. (Be careful though because you can still drown but that is a different matter)
So why does adding salt (or for that matter any thing that dissolves in water) increase the density of water? Remember what density is? It is how compact a substance is. Water is not very compact. At a molecular level, between each water molecule there is some space. When you dissolve salt in water it goes and fills up the spaces between the water molecules. Even though you can’t see the salt grains anymore, it is still there and adds mass to the water which in turn increases it density.
Now going back to our egg experiment you can check this. Measure the weight of the water in the glass before you added the salt. Measure the weight of the salt every time before you add it to the water and make a note of it. At the end once the egg starts to float add up all the weight of he salt. Now measure the end weight of the water (after you added ALL the salt to it). Remember to take out the egg before you measure the weight. You will find that the weight of the water has increased and it would increase by approximately the same amount as the amount of salt you added to it. That is weight of water + weight of salt = weight of salt water. And this tells you that you increased the density of water by adding salt to it.
It is the similar with sea water. Sea water has dissolved particles in it which increases it density and allows the swimmer to float/ swim more easily in sea water than fresh water.
Sorry for the long answer. Again I was not sure of how well you knew the concepts of buoyancy and density, hope this helped and did not confuse you too much!