<b>Drag</b> resists the movement of our bodies through any medium and
is increased by both higher density and viscosity.
Air is much less dense than water and is also less viscous.
So moving through it causes less drag.
Think about walking in a shallow swimming pool.
The greater density and
viscosity of water means it's much more difficult than walking on the sidewalk.
Cream is denser and more viscous than both air and water and
honey is the densest and most viscous of all four.
So, the easiest substance to walk through is air which causes
the least drag followed by water, cream, and honey, which causes the most drag.
There are two types of drag that an animal experiences in the water,
inertial drag and viscous drag and
combined these two types of drag form the third aspect of the aquatic problem.
<b>Inertial drag</b> is caused by the high density of the water.
As a body moves through water,
it disturbs the water molecules, forcing the water to flow around it.
Once the body has passed through the water,
it leaves an empty space where the body used to be.
Water enters this space, swirling and spinning and creating turbulence.
The empty space which pulls the water in to fill it,
also pulls on the object that created it.
This causes inertia drag, sucking the object backwards.
To minimize it, a body should be shaped so
that it disturbs the water as little as possible as it passes through.
Less disturbance means less swirling water, less suction,
and therefore, less wasted energy and decreased speed.
An object that decreases inertial drag by minimizing
disturbance to the water is described as <b>streamlined</b>.