# Drag

## drag.jpg

The fluid force acting on an object can be broken into two components.  One component is in the direction opposite of the direction of the fluid velocity and is called the drag force.  The component of force perpendicular to the direction of the fluid velocity is called the lift.

"Mechanical Potato fluid dynamic forces". Licensed under CC BY-SA 3.0 via Wikimedia Commons.

When an object moves though a fluid there is a force exerted on the body by the fluid. Since force tends to slow the object down, it is called the “drag” force. The drag force acts in the direction opposite to the relative velocity between the fluid and the body. The drag force depends on the shape and size of the body, as well as the density, viscosity, and velocity of the fluid.

An equation that relates the drag force to various fluid and geometric quantities can be determined using the Buckingham P-theorem.  The drag force equation obtained via the Buckingham

-theorem is

where

F is the drag force,

A is the projected are  normal to the direction of flow,

ρ is the fluid density,

V is the velocity of the fluid relative to the object,

Re is the Reynolds number, and

CD is the drag coefficient.

Note that the drag coefficient is a function of the Reynolds number.

## Learning Objectives

Learn how to compute the drag force.

Learn how the drag coefficient depends on the Reynolds number.

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#### Dimensional Analysis of Drag

The drag force equation is one of the most commonly used equations in fluid dynamics.