# Drag - the mechanical force generated when an object moves through a fluid

Drag is the aerodynamic force that opposes an aircraft's motion through the air or a car's motin through the air as it drives along..

It is generated by every part of the vehicle (even the engines!).

The faster you go and the more air particles you intercept the greater the drag is....

### What is 'drag'?

Drag is a mechanical force that is created by the interaction of a solid body as it moves through a fluid (liquid or gas). As the body moves through the fluid it 'bumps into particles of the fluid'.

For drag to be generated:

the solid body must be in contact with the fluid.

If there is no fluid, there is no drag.

there must be motion between the object and the fluid.

If there is no motion, there is no drag.

It makes no difference whether the object moves through a static fluid or whether the fluid moves past a static solid object - al you need is contact and movement.

Drag is a force and is therefore a vector quantity having both a magnitude and a direction.

Drag acts in a direction that is opposite to the motion of the body. It is a form of friction - friction with the fluid particles.

### For example an Aeroplane

We can think of drag for an aircraft as aerodynamic friction, and one of the sources of drag is the skin friction between the molecules of the air and the solid surface of the aircraft. Because the skin friction is an interaction between a solid and a gas, the magnitude of the skin friction depends on properties of both solid and gas.

For the solid, a smooth, waxed surface produces less skin friction than a roughened surface.

For the gas, the magnitude depends on the viscosity of the air and the relative magnitude of the viscous forces to the motion of the flow.

Drag is the also the aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the aircraft and is called form drag. A streamlined body will experience less drag than a blunt awkward shaped one. This is because as air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body.

We can determine the magnitude of the force by integrating (or adding up) the local pressure times the surface area around the entire body (something you may do at Uni - but not at GCSE or A Level). The component of the aerodynamic force that is opposed to the motion is the drag; the component perpendicular to the motion is the lift. Both the lift and drag force act through the center of pressure of the object.

When drag = thrust you have terminal velocity for that 'thrust' force.