What is terminal velocity? Do different objects have different 'terminal velocity' speeds, or is it based on
gravity/friction?

Asked by: Henry Young

Answer

As an object moves through a fluid, such as a liquid or gas there exists a drag force
that acts to impede the motion. This drag force not only acts in the direction
opposite to the motion but also depends on the square of the speed of the object.
The fact that the drag force increases with speed has an important consequence.
Consider a falling body. It is acted on by gravity and a drag force due to the air.
When the body begins to fall the speed is slow and thus its drag force is also
small. The acceleration of the falling body is g.
As the body's speed increases so too does the drag and at some point the speed would
increase tot he point where the drag force would act to cancel out the force due to
gravity. Thus the body no longer accelerates and the speed remains constant.

Different objects would have different terminal speeds. The terminal velocity is not
only dependent on the speed of an object but also the density of the fluid through
which the object moves, the cross sectional area presented by the moving object and
a drag coefficient. The cross sectional area is the area cutting across the object in
a plane perpendicular to the direction of motion. The drag coefficient is a
dimensionless constant and depends on the shape of the object.
Thus different objects would have different terminal velocities depending on their
shape and the cross sectional area they present moving through the medium as well as
the acceleration due to gravity.
Answered by: David Latchman, B.Sc. Physics, University of the West Indies

'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'