Dynamic friction refers to the friction experienced by an object when it is in motion. It is the resistance encountered by an object as it moves against another surface. This type of friction is caused by the microscopic irregularities present on the surfaces that come into contact with each other. When an object is in motion, these irregularities interact and create a resistance force that opposes the motion.

Causes of Dynamic Friction:
Dynamic friction occurs due to various factors, including:
1. Surface roughness: The roughness of the surfaces in contact affects the amount of friction. Smoother surfaces tend to have less friction, while rough surfaces create more resistance.
2. Pressure: The force exerted between the surfaces also affects the friction. Higher pressure increases the frictional force.
3. Lubrication: The presence of a lubricant between the surfaces can reduce the friction by creating a barrier that minimizes the contact between rough surfaces.

Characteristics of Dynamic Friction:
1. Magnitude: The force of dynamic friction is generally greater than the force of static friction, which is the friction between an object at rest and the surface it is resting on.
2. Direction: The dynamic friction force acts in the opposite direction to the motion of the object. It opposes the applied force and tries to bring the object to a stop.
3. Proportional to Normal Force: The force of dynamic friction is proportional to the normal force between the object and the surface. The greater the normal force, the greater the friction force.

Examples of Dynamic Friction:
Dynamic friction can be observed in various everyday situations, such as:
1. Sliding objects: When a book slides on a table, the dynamic friction between the book and the table's surface slows down its motion.
2. Rolling objects: When a ball rolls on the ground, the dynamic friction between the ball and the ground resists its rolling motion.
3. Vehicles on the road: The tires of a car experience dynamic friction with the road surface, which provides the necessary traction for motion.

Importance of Dynamic Friction:
Understanding dynamic friction is crucial in various fields, including engineering, transportation, and sports. It helps in designing efficient machinery, optimizing vehicle performance, and enhancing the safety of sports activities like braking in car racing or gripping in basketball.

In conclusion, dynamic friction refers to the friction experienced by an object when it is in motion. It is caused by the microscopic irregularities on the surfaces in contact and acts in the opposite direction to the object's motion. Understanding dynamic friction is essential for various practical applications and plays a significant role in everyday life.