Friction Summary Class 8 NCERT Summary Chapter 9

Introduction

The vehicles, any object, moving over the surface of another object slows down due to the force of friction applied on it.

Friction

• It is an opposing force that acts between surfaces in contact moving with respect to each other.
• It always opposes relative motion between two surfaces.

Cause of friction

• Friction is cause by the irregularities on the two surfaces in contact.
• We are able to walk because of the force of friction.

Nature of surfaces for Friction

• Smooth surfaces: less friction
• Rough surfaces: greater friction

Friction when object is pressed hard

• Greater pressing force means Greater friction.

Friction according to mass of object

Greater mass: Greater friction

• Sliding friction < Static friction
• Rolling friction < Sliding friction

Examples where friction is useful

(i) Walking
(ii) Handling any object
(iii) Rolling motion of ball or wheel

Examples where friction is harmful

(i) Energy dissipation of engine due to friction between surfaces in motion.
(ii) More energy is lost in pulling or pushing an object in rough surfaces.
(iii) Wear and tear of shoe soles and tyres.(iv) Lubrication: Powder on carom board, oil in machine
(v) Wheel: Wheels reduce friction (because rolling friction < sliding friction).

(vi) Shoe soles and tires are threaded to increase friction for a better grip.

(vii) Fluid friction is minimised by giving suitable shapes to vehicles moving through fluids.

Rolling Friction

• When an object rolls over another surface, the resistance to its movement is called rolling friction.
• Rolling friction is less than sliding friction, which is why machines often use ball bearings to replace sliding motion with rolling.

Sliding Friction

• When an object slides across another surface, the resistance is known as sliding friction.
• Rolling friction is lower than sliding friction.
• Ball bearings help convert sliding friction into rolling friction.

Fluid Friction

• Fluid friction can be reduced by designing vehicles with shapes that move more easily through fluids.
• Fluid friction is also referred to as drag.
• The force of friction caused by fluids is known as drag.
• The frictional force acting on an object in a fluid depends on its speed relative to the fluid.