What is Friction Class 8: Definition, Types & Examples Explained

Definition of Friction for Class 8 Students

Friction is a force that resists the motion of one surface sliding over another. It acts in the opposite direction to the movement or intended movement. In Class 8 NCERT Science, friction is introduced as a force that arises due to the roughness of surfaces in contact.

• When you try to push a heavy box, friction between the box and the floor resists the push.
• Friction acts parallel to the surfaces in contact.

Formula related to friction:

The force of friction ($F_f$) is proportional to the normal force ($N$) between the surfaces:

$$F_f = \\mu \, N$$

where $\\mu$ is the coefficient of friction, which depends on the materials of the surfaces.

Understanding friction helps explain why objects slow down or why it is hard to move heavy things.

Types of Friction Explained with Examples

Friction is classified into four main types based on the nature of the surfaces and the motion involved:

1. Static Friction: Acts when an object is at rest and prevents it from moving. Example: A book resting on a table. 2. Sliding Friction: Occurs when one surface slides over another. Example: A box sliding on the floor. 3. Rolling Friction: Happens when an object rolls over a surface. Example: A ball rolling on the ground. 4. Fluid Friction: Occurs when an object moves through a fluid (liquid or gas). Example: A swimmer moving in water.

Each type plays a different role in daily life and machines.

Factors Affecting Friction in Everyday Life

Several factors influence the amount of friction between two surfaces:

• Surface Roughness: Rougher surfaces have more friction due to more contact points.
• Normal Force: Greater force pressing the surfaces together increases friction.
• Type of Materials: Different materials have different coefficients of friction.

For example, walking on a smooth ice surface is harder because friction is less, while walking on rough ground is easier due to higher friction.

Worked Example:

If a block weighing 20 N rests on a surface with a coefficient of friction $\\mu = 0.3$, the frictional force is:

$$F_f = \\mu \, N = 0.3 \\times 20 = 6 \, \text{N}$$

This means 6 N of force resists the block’s motion.

Advantages and Disadvantages of Friction

Friction has both positive and negative effects in our daily lives.

Advantages:

• Enables walking without slipping.
• Allows vehicles to move and stop safely.
• Helps in writing with a pen or pencil.

Disadvantages:

• Causes wear and tear of machine parts.
• Leads to loss of energy as heat.
• Makes movement harder, requiring more effort.

Understanding these helps in designing machines and choosing materials to control friction effectively.

How to Reduce or Increase Friction: Practical Methods

Sometimes, we want to reduce friction to save energy or increase it for safety.

Ways to reduce friction:

• Using lubricants like oil or grease.
• Polishing surfaces to make them smooth.
• Using ball bearings in machines.

Ways to increase friction:

• Making surfaces rougher.
• Increasing the normal force.
• Using rubber soles on shoes.

Example: Lubricating a bicycle chain reduces friction, making pedaling easier and preventing wear.