Articles

Water Properties (Part 2)


Float or sink?

Ever wonder why some objects sink in water? Why does a nail sink and a ship float on water? What makes water support some objects so that they can float on it?

Understanding why some objects sink in water while others float is very important in building ships, submarines, etc. If a nail sinks into the water and a ship floats, it is clear that it has nothing to do with whether the object is light or heavy, since a nail is a few grams long and a ship weighs tons.

In water we can lift a person with little effort, while out of the water we could not even move them from the ground. This is because water pushes a person's body upwards. The force that water exerts on bodies dipped from the bottom up (like a "push") is called buoyancy.

The amount of water displaced by bodies is an important factor in the floating or sinking of objects. The nail, because it has little volume, displaces a minimum of water when dipped. Since the ship is very bulky, displaces a large amount of water. Then your "weight" is balanced by the force with which the water "pushes" you, that is, by the thrust.


When the thrust (E) equals the weight (P) the object fluctuates, but when the weight is greater than the thrust the object sinks. When the submarine wants to sink it increases its weight by filling its seawater tanks.

Water exerts pressure

Have you ever tried to hold the jet of water coming out of a hose with your finger? What happened? Water prevented by the finger from flowing exerts pressure and comes out harder.

All liquids generally exert pressure. One way to demonstrate the pressure exerted by a "liquid" column is to drill holes in a 2 liter (soda) plastic bottle and fill it with water.

  • The experience illustrated below indicates that the pressure exerted by a liquid increases with depth because the flow rate from the first hole is less than the flow rate from the other two. It can be seen that the greater the depth or height of liquid, the water trickle reaches a greater distance. The pressure is said to be higher and depends on the depth of the orifice considered.

Pressure and diving

When a person dives, they may feel pain in the inner ear. Do you know why this happens? Again, the explanation is related to the pressure that water exerts.

As we dive, as we move to the bottom, the height of the liquid column above us increases. The higher the height of this column, the greater the pressure exerted by the liquid on us. For this reason, in the depths of the oceans the water pressure is high and man cannot get there without pressure protection equipment.