12/20/05
7:24 PM
MAZER
Page 113
Water Pressure and Depth
Pressure in fluids increases with depth. Think back to the example of
the cylinder sitting on the palm of your hand. You slowly pour water into
the cylinder. At first, when the water level is very low, you feel very little
pressure on your palm. As you add more water, the pressure you feel
increases. The more water you add, the stronger the pressure you feel.
If a cylinder of water were resting on your palm,
you would feel the pressure of the water pushing
down on an area of your hand. The deeper the water
column, the greater the pressure. Now imagine going
swimming in a deep pool. The pool is just like a large
column of water. When you swim, water pressure
acts not just on part of your hand, but on your entire
body. However, the principle remains the same. The
more water that pushes on you from above, the
greater the pressure you feel.
At the surface of the pool, you sense very little
water pressure. That is because only a very small
amount of water pushes down on you there. Suppose you dive under
water. The deeper you dive, the more pressure you feel. You are feeling
the weight of more and more water pressing on you.
reading check icon
If you dove to the bottom of a pool, why would you feel more
pressure there than near the surface?
Water pressure increases with the depth of the water. This is because
the weight of the column of water above the object increases. But a
large, shallow pond may have more water in it than a small, deep pond.
Why doesn’t water pressure depend on the volume of water?
Pressure is determined by force and the area over which that force
acts. Suppose two ponds are the same depth. One pond has twice the
water of the other pond, so it is twice as large in area. The water in the
large pond presses on an object with twice the weight of the water in
the small pond. However, the large pond applies that force over twice
the area. The result is that the pressure is the same at the bottom of a
large pond as it is at the bottom of a small pond.
Increasing Pressure
s8pe-10402-ca
A deep-diving whale at
1000 meters (3280 ft)
below the surface
experiences about 34
times more pressure than
a turtle diving to a depth
of 20 meters (65 ft).
The water pressure at a given depth does not depend on the shape
of the container. Suppose you fill a cylinder, a bowl, and a tube with
twists and turns in it all to the same height with water. The water
pressure at the bottom of each of these containers will be exactly the
same. Depth is what determines the pressure for any given fluid.
Chapter 4: Density and Buoyancy 113
PDF