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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 .
