When any object rises or falls through a fluid it will experience a
viscous drag, whether it is a parachutist or spacecraft falling through air, a stone falling
through water or a bubble rising through fizzy lemonade.
Consider an object falling
through a viscous fluid. As the object falls so its velocity increases but so does the drag on it
due to the movement of the fluid past it. We call this the viscous drag.
As the object
falls faster and faster the drag force increases. Eventually the drag force increases to a value
where it is equal to the weight of the object and the body continues to fall at a steady speed.
We call this the terminal velocity of the object. (shown
by Figure 2).
For details of the mathematics of this force see: Stokes'
Law
At this speed:
Large raindrops hurt much more than small ones when
they fall on you - it's not just that they are heavier, they have a higher terminal velocity and so
are actually falling faster.
People falling through the atmosphere will also eventually
reach their terminal velocity. For low-level air (below about 3000 m) this is around 200
km/hour flat out and just over 320 km/hour head down. However at high altitudes around 30
000m this can reach almost 1000 km/hour!
Figure 3 shows how the velocity of an
object will increase with time as it falls through a viscous fluid.
