# What causes drag

1. May 18, 2008

### flash9286

Sorry if this is a stupid question but can someone give me a quick conceptual overview of what causes drag.

I was thinking that it is caused by the object moving through the fluid collides with the fluid particles and by newton's 3rd law the fluid particles impart a force on the object in opposition to the direction the object is moving. But I have no idea if this is actually right and googling didn't help.

2. May 18, 2008

### Astronuc

Staff Emeritus
Your intuition is correct, more or less. At the surface of an object, the fluid (air/liquid) is still, effectively attached to the surface. As an object moves through a fluid it pushes into the fluid and the fluid is deflected around the object. The object looses momentum to the surrounding fluid.

3. May 19, 2008

### rcgldr

Depending on the shape of the object, Coanda and "void" effects cause acceleration of air which causes drag.

Imagine a bus at speed. At the front of the bus the air can seperate and flow around the bus, but there is still forwards acceleration at the front of the bus. The back of the bus introduces a moving low pressure void as it passes through a volume of air, and this accelerates air forwards. In the case of the bus, most of the acceleration of air occurs at the back of the bus.

In the case of a sphere, the air seperates and flows around the front half of the sphere, but because of friction between the air and surface of the sphere, and friction within the air itsleft (viscosity), and initially void effect during increases in speed, the air continues to curve part way around the back half of the sphere before the flow becomes detached and transitions into turbulent flow. The turbulent flow creates eddies and these may reattach to the back half of the sphere because of "void" effect. In the case of a sphere, more of the drag occurs behind than in front of the sphere.

A modified tear drop shape reduces drag, by gradually introducing the "void" at the "rear" of the shape that requires much less forwards acceleration of air.