Friction Class 8 Science Chapter 12 Notes

Key Terms in Friction

Friction

The force that opposes the relative motion between two surfaces in contact.

Static Friction

The friction that exists between two surfaces when an object is at rest. It is the force that you must overcome to start moving an object.

Sliding Friction (Kinetic Friction)

The friction that exists when one surface slides over another. It is slightly less than static friction.

Rolling Friction

The friction that occurs when an object, such as a ball or wheel, rolls over a surface. It is the weakest type of friction.

Fluid Friction (Drag)

The frictional force exerted by fluids (liquids and gases) on objects moving through them.

Lubricants

Substances that are applied to surfaces to reduce friction, such as oil, grease, or graphite.

Interlocking

The catching or locking of irregularities (hills and valleys) on two surfaces that are in contact, which is the primary cause of friction.

What Causes Friction?

Friction is a contact force that arises primarily due to the irregularities on the surfaces of two objects in contact. Even surfaces that appear very smooth to the naked eye have microscopic hills and valleys. When one surface is placed on another, these irregularities get interlocked. To move one surface over the other, we must apply enough force to break this interlocking. The resistance to this movement is what we call friction. The strength of this interlocking depends on two main factors. First is the nature of the surfaces – rougher surfaces have larger irregularities and thus create more friction. Second is the force pressing the surfaces together (also known as the normal force). If you press down harder on an object, the irregularities are forced into each other more deeply, increasing the interlocking and therefore increasing the frictional force. This is why it's harder to push a heavy box than a light one across the same floor.

Static vs. Sliding vs. Rolling Friction

Must-Remember Points about Friction

• Friction always opposes the direction of motion or attempted motion.
• Friction depends on the nature of the surfaces in contact (e.g., rough surfaces have more friction than smooth ones).
• Friction depends on how hard the two surfaces are pressed together. More force means more friction.
• Friction is independent of the area of contact between the two surfaces.
• Static friction is self-adjusting. It increases with the applied force until it reaches its maximum limit.
• Sliding friction is slightly smaller than the maximum static friction.
• Rolling friction is the smallest of the three main types of friction.
• Friction can be a necessary evil: it's useful for walking and gripping, but causes wear and tear and energy loss.
• Friction can be reduced by using lubricants, polishing surfaces, or using ball bearings.
• Fluid friction (drag) depends on the speed of the object, its shape, and the nature of the fluid.

Friction: A Necessary Evil

Exam Writing Tip

When asked to explain factors affecting friction, always mention both the nature of surfaces and the force pressing them together. A common mistake is to say friction depends on the 'area of contact'—this is incorrect for solid friction at this level. Also, when solving problems or explaining concepts, draw a simple box diagram and use arrows to show the direction of applied force and the opposing frictional force. This makes your answer clearer and can fetch you full marks.

Quick Revision Check

• Q: Why are the soles of our shoes grooved? A: The grooves increase the roughness of the soles, which increases friction between the shoes and the ground. This provides a better grip and prevents us from slipping.
• Q: A boy is pushing a heavy box that is not moving. Which type of friction is acting on the box? A: Static friction is acting on the box. It is opposing the boy's push and preventing the box from moving.
• Q: Why is it easier to roll a suitcase with wheels than to drag it? A: It is easier because rolling friction is much smaller than sliding friction. The wheels replace sliding with rolling, reducing the force needed to move the suitcase.
• Q: What is the special name for the frictional force exerted by fluids? A: The frictional force exerted by fluids (like air and water) is called drag.

Frequently Asked Questions on Friction

Frequently Asked Questions