Answer: Friction is a force that opposes the relative motion between two surfaces in contact.

Answer: Friction is a contact force.

Answer: Friction acts in a direction opposite to the direction of motion or attempted motion.

Answer: Walking on the ground is possible due to friction between our shoes and the ground.

Answer: It comes to rest because friction between the ball and the ground opposes its motion.

Answer: The force of friction between brakes and the wheel stops the bicycle.

Answer: An external force greater than the frictional force is needed to overcome friction.

Answer: Because friction is both useful (helps us walk, write, etc.) and harmful (causes wear and tear, heating), so it is necessary but sometimes undesirable.

Answer: The main cause of friction is the interlocking of microscopic irregularities on the surfaces in contact.

Answer: No, even surfaces that look smooth have tiny irregularities when viewed under a microscope.

Answer: Rough surfaces increase interlocking of irregularities, so friction becomes larger.

Answer: Smooth surfaces reduce interlocking and hence decrease friction.

Answer: Friction depends on (i) the nature of surfaces (rough or smooth) and (ii) how hard the surfaces are pressed together (normal force).

Answer: If normal force increases, friction also increases.

Answer: No, different pairs of surfaces (wood–wood, metal–metal, rubber–road, etc.) have different amounts of friction.

Answer: We can polish surfaces or apply lubricants like oil or grease to make them smoother and reduce friction.

Answer: Static friction is the frictional force that acts on a body at rest and prevents it from starting to move.

Answer: Static friction acts when an external force tries to move an object but the object is still at rest.

Answer: Sliding friction is the frictional force that acts when one surface slides over another.

Answer: A book sliding on a table is an example of sliding friction.

Answer: Rolling friction is the frictional force that acts when one body rolls over another surface.

Answer: The friction between the tyres of a car and the road when the car is moving is rolling friction.

Answer: Static friction is greater than sliding friction for the same pair of surfaces.

Answer: Rolling friction is smaller than sliding friction.

Answer: It is easier to roll a box because rolling friction is much less than sliding friction.

Answer: Static friction > Sliding friction > Rolling friction.

Answer: Friction between our feet and the ground provides grip, so we can push the ground backwards and move forward.

Answer: Rubber with grooves increases friction with the road, preventing slipping and improving control.

Answer: Friction between the pencil/pen tip and paper allows them to leave marks on the paper.

Answer: Rough surfaces offer more friction, giving better grip to our fingers or hands.

Answer: Brakes create friction with the wheels, which opposes motion and stops the vehicle.

Answer: Friction between the surfaces of the rope prevents it from slipping, so knots hold firmly.

Answer: Friction between chalk and blackboard causes chalk particles to stick, forming visible writing.

Answer: Friction converts part of mechanical energy into heat, leading to energy loss.

Answer: Tyres of vehicles wear out over time due to friction with the road.

Answer: To reduce friction, heating and wear of machine parts, and to make them run smoothly.

Answer: Rubbing hands together in winter produces heat due to friction and keeps them warm.

Answer: Friction between the matchstick and the rough surface produces heat, igniting the chemicals on the match head.

Answer: Lubricants are substances like oil or grease applied between surfaces to reduce friction.

Answer: Lubricants fill up the irregularities between surfaces and make them smoother, thus reducing friction.

Answer: Ball bearings reduce sliding friction by changing it into much smaller rolling friction.

Answer: Polishing makes a surface smoother and reduces friction between surfaces.

Answer: Roughening the surfaces, such as making treads on tyres, is one method to increase friction.

Answer: Shoe soles are made rough to increase friction with the ground and prevent slipping.

Answer: Fluid friction is the friction experienced by objects moving through fluids (liquids or gases).

Answer: Air and water both offer fluid friction.

Answer: Drag is another name for fluid friction, the force that opposes motion of an object through a fluid.

Answer: Fluid friction depends on the object’s speed, its shape and the nature (thickness) of the fluid.

Answer: A streamlined shape is narrow in front and wider in the middle, tapering at the back, to reduce fluid friction.

Answer: Fishes and birds have streamlined bodies so that they can move easily through water or air with less fluid friction.