**Drag Force:**

The frictional force exerted by fluids is called drag force. It is a resistance force that opposes the motion of an object through a fluid (liquid or gas). When an object moves through a fluid, it experiences drag force due to the interaction between the object and the fluid particles.

**Explanation:**

When an object moves through a fluid, the fluid particles exert a force on the object in the opposite direction to its motion. This force is known as the drag force or fluid friction. The drag force depends on several factors including the shape and size of the object, the speed of the object, and the properties of the fluid.

**Factors Affecting Drag Force:**

1. **Shape and Size of the Object:** The shape and size of the object greatly influence the drag force. Objects with a streamlined or aerodynamic shape experience less drag because they cause less disturbance to the fluid flow. On the other hand, objects with irregular shapes or larger sizes experience more drag.

2. **Speed of the Object:** The speed at which the object moves through the fluid also affects the drag force. As the speed increases, the drag force also increases. This is because at higher speeds, the fluid particles collide more frequently with the object, resulting in a larger drag force.

3. **Properties of the Fluid:** The properties of the fluid, such as its viscosity and density, also play a role in determining the drag force. Viscous fluids, like honey or syrup, create more resistance and therefore have higher drag forces compared to less viscous fluids, like water or air.

**Importance of Drag Force:**

Drag force is an important concept in various fields such as aerodynamics, fluid dynamics, and transportation. It helps engineers design efficient vehicles and structures that can minimize drag and maximize speed and efficiency. Understanding drag force is also crucial in sports, such as swimming and cycling, where athletes try to minimize drag to increase their performance.

In summary, the frictional force exerted by fluids is called drag force. It is a resistance force that opposes the motion of an object through a fluid. The drag force depends on factors like the shape and size of the object, the speed of the object, and the properties of the fluid.