A drag force is the resistance force caused by the motion of a body through a fluid, such as water or air. A drag force acts opposite to the direction of the oncoming flow velocity. This is the relative velocity between the body and the fluid.
The drag force D
exerted on a body traveling though a fluid is given by
is the drag coefficient, which can vary along with the speed of the body. But typical values range from 0.4 to 1.0 for different fluids (such as air and water)
is the density of the fluid through which the body is moving
is the speed of the body relative to the fluid
is the projected cross-sectional area of the body perpendicular to the flow direction (that is, perpendicular to v
). This is illustrated in the figure below
To see an example of a problem that involves drag, go to The physics of skydiving
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