1. Whenever things are rolling over the ground, they stop after moving sometime. Give me another instance when on its own an object stopped moving. Carrom coins

2. Activity of two students to show opposing forces. An opposing force always opposes ___________ In a rolling object motion is opposed therefore there is some kind of opposing force is acting. How many surfaces are affected by this force? In which region/ area is the force experienced? What condition should the two surfaces satisfy? Another word for rubbing is ____________.

3. How many surfaces are affected by this force? Two In which region/ area is the force experienced? Between the two surfaces. What condition should the two surfaces satisfy? The two surfaces are in contact with each other. Understanding Friction

4. Ch: Friction What opposes motion? How many surfaces are being rubbed? How are the surfaces ? Friction is that force which opposes the relative motion of between the two surfaces that are in contact with each other. Word friction is derived from a Latin word `fricare’ which means `to rub’.

5. Motion Friction Pushing Force Friction always opposes the motion between two bodies irrespective of the direction of motion

6. Properties of Friction 1.It is a force that opposes motion. 2.It always produces heat. 3.It causes wear and tear. Factors which affect the force of friction. 1.Depends on smoothness of the surface. 2.Solids, liquids and gases all tends to stop things moving in or over them. 3.Depends on weight of the body. 4.Has no link with the area of the surface in contact (as mass/ weight of an object remains same)

7. Types of Friction Static Friction Sliding Friction Rolling Friction Friction is a self adjusting force. It increases at the same rate as the applied force, when it is gradually increased At a certain stage the body just begins to move. At this stage static friction Fs is maximum. An rolling object like a ball or vehicles moving on wheels experiences rolling friction which is produced between the rolling object and the surface. With a sliding object, friction is produced between the object and the surface called sliding friction.

10. A smooth glass slab produces more friction than a rolling object because a sliding object is obstructed by more force due to this reason the sliding object faces more friction than a rolling object. Hence to move heavy objects we use rollers. Infact sliding is replaced in all machines by rolling by the use of ball bearings. Also note that, the force required to keep a body in motion is less than the force required to start the motion.

11. Advantages and disadvantages of friction Walking or slipping. Writing activity, Sawing of wood, burning of match stick, holding nail in a wall, Studs of football boots are activities which need friction to happen. Wheel and break friction cause slows down vehicles. Road and tyres friction cause moving of vehicles. Friction between Brake lining and drum of the wheel – result in applying brakes of automobiles. Without friction no building could be constructed. Hence frictionless life would have had many difficulties.

12. Ways to increase friction in areas where friction plays a decisive role. Soles of shoes and tyres of automobiles are made corrugated and rough, resulting to increase friction in order to provide a better grip on the ground-road to prevent skidding. The brake pads/linings in automobiles are made rough to increase friction between the lining and rim/ drum of the wheel. Wrestlers and kabaddi players rub their hands in soil for better and firm grip due to increased friction between their hands and opponent’s body.

13. Disadvantages of Friction Opposes motion and ultimately stop motion. Also reduces efficiency of machines as energy is wasted in overcoming friction. Causes wear and tear. Things like tyres, shoes wear out. Produces heat. Machines on heating get damaged. Have to be occasionally replaced or undergo mechanical cleaning which means financial loss. Friction during walking causes you to slip over a banana skin lying on the floor as banana skin reduces friction between your foot and the road. Causes wastage of energy : Vehicles on a kuccha road or bumpy road waste energy and get heated up as engine exerts more force in moving a car due to more friction between the road and the wheels, hence increasing the expenditure on fuels.

14. Methods to reduce friction 1.By using lubricants (grease/ oil/ graphite powder/ compressed air are used in machines) the unevenness of the two surfaces get removed, due to the formation of a thin smooth layer between the two surfaces and they roll easily over each other. 2.By using ball bearings : Small lead shots (ball bearing) are introduced in between the two moving surfaces to reduce friction. Rolling friction is lesser than sliding friction 3.By varnishing and electroplating surface roughness can be reduced and polished surface reduce friction. 4.By using wheels and rollers : Wheels reduce the friction hence conveyor belts move on rollers or wheels, suitcases are fitted with wheels. 5.By streamlining – A streamlined shape reduces friction when the body moves through water or air. Boats, fishes, birds, aircraft. The symmetrical or streamlined shape offers least resistance. 6.By sprinkling a soft slippery fine powder on the surfaces – carrom board.