Class 8 ppt 2 friciton
- 1. LEARNING GOALS VIDEO : II • STATIC FRICTION • SLIDING FRICTION • ROLLING FRICTION • FLUID FRICTION
- 2. STATIC FRICTION • Force required to overcome friction at instant an object starts moving from rest is called Static Friction .
- 3. SLIDING FRICTION • Sliding Friction is less than static friction . • When one surface is sliding over the other surface, sliding friction comes into play.
- 4. ROLLING FRICTION • When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction.
- 5. ROLLING FRICTION • Ball Bearings reduce this friction . They are used between hubs and the axles of ceiling fans and bicycles.
- 6. FLUID FRICTION • The frictional force exerted by fluids is also called drag . Fluid Friction can be minimised by giving suitable shapes to bodies moving in fluids. • To overcome fluid friction bodies of fish and birds are streamlined. Similarly, the ships and aeroplanes are also made streamlined.
- 7. EXTRA QUESTIONS Q : Give three instances when friction produces noticeable heat ? A : The instances are (1) Rubbing your hands together to keep warm in winter . (2) Rubbing sticks to make fire . (3) Parts of car engine rubbing together cause it to overheat . Q : Name two factors on which magnitude of frictional force depends A : (1) Friction depends on nature of surface in contact . (2) Friction depends on how hard the two surfaces press together . Q : What is use of ball bearings ? A : Use of ball bearing is to reduce the friction . Q : Why are soles of shoes and tyres of cars etc grooved ? A : It is done to provide the shoes a better grip on the floor , so that one can move safely . Similarly tyres of cars provides better grip with ground .
- 8. EXTRA QUESTIONS Q : List any three methods of reducing friction A : (1) by applying lubricant (2) by using ball bearing (3) by polishing surfaces Q : Name special shape given to objects to move in fluids A : Streamlined Shape Q : You would not have forgotten the incident of Astronaut Kalpana Chawla . What was the reason behind that ? A : When spacecraft re-enters the atmosphere , its surface gets extremely hot due to friction between air and spacecraft that is moving at high speed . To protect the craft its outer surface is covered with tiles of special material , that can tolerate high heat . The reason of burning of spacecraft in which Kalpana Chawla was travelling was the heat by friction
- 9. EXTRA QUESTIONS Q : What are Lubricants ? A : Substances that reduce friction Q : What is Interlocking ? A : Irregularities on two surfaces in contact may clasp with each other , increasing friction .
- 10. SUMMARY • ROLLING FRICTION • SLIDING FRICTION • FLUID FRICTION • STATIC FRICTION • EXTRA QUESTIONS