Pressure
is a force exerted on an area. In 1647, French scientist Bliss Pascal
(1623-1662) discovered that water
exerts the same pressure in all directions. This statement is called Pascal's
Principle.
To
understand Pascal's principle, think of a small cube of water.
Two forces act on the cube, as a result the total downward force on the cube is
greater than the upward force.
Now,
according to Newton's third law, the outward force of water will be equal to
the inward force of water.
What does that force do? If the cube is placed on the edge of a box, then the
box will exert a force.
You
can also experiment with this process yourself. Use an empty container of water
(any juice or milk). Make a hole on
one side. Pour water into the box. You apply force with your thumb to keep the water
coming out of the box. If the hole is
at the bottom of the box, the same situation will happen. What will the force
acting downward do to the top of the cube? If the cube is not on the upper
surface, then the upward force will be exerted by the water. If it is upward,
then the air pressure will exert this force.
Since
the downward force exerted by the water is
greater than the upward force, then the force and pressure will also increase
with the increase in the depth of the water, so the bottom of a water
box will have more pressure than the upper part. The increasing amount of
pressure can be determined by multiplying density (ρ), gravity (g) and depth
(h). This means that the pressure at the bottom will be Pbottom = Ptop + ρgh.
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