Motion in Two and Three Dimensions
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Drag Forces: |
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Drag is a mechanical force. It is generated by the interaction and contact of a solid body with the
fluid. The fluid can be either a liquid or a gas. It is not generated by a force field where two objects
can affect each other without being in physical contact. For the generation of a drag force, the
object must be in contact with the fluid. If there is no fluid then there would
be no drag force. The
drag force is generated by the differences in the velocities of the object and the fluid in contact.
There must be a relative motion between the object and the fluid. If there is no
relative motion there
is no drag. There is no difference in either of the cases like: whether the object moves through the
static fluid or the fluid moves past the static object. In either of the cases the drag force would
come into play. The examples of such kind of situations are: an aircraft moving through the air,
submarine moving inside the sea.
The drag force is made up of friction forces, which act parallel to the surface of the object and
the pressure force, which acts perpendicular to the surface of the object. For a solid object moving
through a liquid or gas, drag is the sum of all the aerodynamic or hydrodynamic forces in the
direction of the external fluid flow. It should be noted that the drag force opposes the motion of the
object.
The drag force is approximately proportional to some power of the speed of the object.
When an object falls from rest, its velocity will be
where m is the mass of the object, t is the time,
b = 6πηr, r is the Stoke’s radius of the particle,
and η is the fluid viscosity. Then the Stoke’s drag force
would be F = -bv
At high velocity, the total drag force (Fd) on a body is given by
Where ρ = density of the fluid, v = speed of the
object relative to the fluid, A is the reference area,
and Cd is the drag coefficient
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Newtonian Mechanics