# Viscous force

## Definition

QUICK PHRASES: Fluid friction, resistance to motion through fluid

The **viscous force** is the force between a body and a fluid (liquid or gas) moving past it, in a direction so as to oppose the flow of the fluid past the object. In particular, the force acts on the object in the direction in which the *fluid* is moving relative to it (and hence, opposite to the direction in which *it* is moving relative to the fluid).

Viscous force is an analogue in fluids of the force of friction.

## Fluid models for viscous force

Type of fluid | Brief description of model for viscous force |
---|---|

ideal fluid | no viscous force |

Newtonian fluid | constant viscosity, i.e., magnitude of viscous force is proportional to the speed (or the rate of shear). |