09. Class 8 Science Chapter 9 Friction

Definition: Friction is the force that opposes the relative motion between two surfaces in contact. Imagine pushing a book across a table – friction acts like an invisible hand, trying to hold it back.

Dependence: Friction depends on three key factors:

• Nature of surfaces: Rough surfaces have more friction than smooth surfaces. Imagine pushing a brick on sandpaper versus a sheet of glass.
• Smoothness of surfaces: Smoother surfaces have less friction.
• Force pressing the surfaces together: More pressure means more friction. Think about pushing a book with one finger versus your entire hand.

Activity 9.1: This activity demonstrates the force of friction by pushing a book on a table. Notice how the book eventually stops due to friction.

Direction: Friction always acts in the opposite direction of the applied force. Push right, friction pushes left. Pull left, friction pulls right.

Surface Dependency: Friction acts between the surfaces of two objects in contact. In Activity 9.1, it acts between the book’s bottom and the table’s surface.

Activity 9.2: This activity uses a spring balance to measure friction between a brick and different materials. Observe how the force of friction changes depending on the material.

Observations: Rough surfaces like sandpaper result in greater friction compared to smoother surfaces like wood. This is because rough surfaces have more interlocking irregularities.

Activity 9.3: This inclined plane activity demonstrates the impact of different surfaces on friction.

Observations: The brick stops sooner on the rougher surface due to increased friction caused by its irregularities. Smoother surfaces have less friction, allowing the brick to slide further.

Examples: Friction plays a crucial role in many activities like walking (gripping the ground), writing (pencils on paper), and fixing objects (screws holding things together). However, it also wears out materials like brakes and tires.

Static vs. Sliding Friction: Static friction is the force needed to start moving an object (it’s higher). Sliding friction is the force opposing ongoing motion (it’s smaller). Imagine pushing a heavy box – it takes more effort to get it moving than to keep it moving.

Activity 9.4: This activity uses pencils and a book to demonstrate how rolling reduces friction. Observe how the book with rolling pencils moves further than the book with stationary pencils.

Treaded Surfaces: Soles of shoes and vehicle tires are designed with treads (grooves) to increase friction for better grip on slippery surfaces.

Lubricants: Oil, grease, and graphite reduce friction by creating a slippery layer between surfaces. This is why wheels use bearings lubricated with oil or grease.

Rolling Friction: Rolling reduces friction significantly compared to sliding. This is why cars have wheels and not giant skis!

Advantage: Rolling friction is much smaller than sliding friction, making it more efficient for movement.

Fluids: Air, water, and other liquids exert friction called drag on objects moving through them. Feel the drag when you swim or bike fast.

Shapes in Nature: Many objects in nature, like bird wings and fish bodies, have streamlined shapes to minimize fluid friction and move efficiently through air or water.