CBSE Class 8, NCERT Science
Chapter 12, Friction
Question.1: Fill in the blanks:
(a) Friction opposes the ________ between the surfaces in contact with each other.
(b) Friction depends on the ________ of surfaces.
(c) Friction produces ________.
(d) Sprinkling of powder on the carom board _______ friction.
(e) Sliding friction is __________ than the static friction.
Answer: (a) relative motion (b) nature (c) heat (d) reduces (e) less
Question.2: Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Choose the correct arrangement:
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
Question.3: Alida runs her toy car on a dry marble floor, wet marble floor, newspaper and towel spread on the floor. What will be the force of friction in increasing order, acting on the car on different surfaces?
Answer: wet marble floor, dry marble floor, newspaper and towel.
Question.4: Supposing your writing desk is tilted little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Answer: Frictional force is acting opposite to the movement of the book i.e. upwards.
Question.5: You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer: It is difficult to walk on a soapy floor because layer of soap makes floor smooth. The coating of soapy water reduces the friction and the foot can not make a proper grip on the floor and it starts getting to slip on the floor.
Question.6: Explain why sportsmen use shoes with spikes.
Answer: Sportsman use shoes with spike to increase the friction so that the shoes do not slip while they run or play.
Question.7: Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer: Seema will have to apply larger force since; she will experience more frictional force because heavy object will be pressed hard against the opposite surface and produces more friction.
Question.8: Explain why sliding friction less than static friction.
Answer: Sliding friction is always less than static friction because two sliding objects find less time to get interlocked against each other’s irregularities of surfaces as a result of which they experience less friction.
Question.9: Give examples to show that friction is both a friend and foe.
Friction is a friend because -
· It allows us to walk comfortably.
· It allows us to grip and catch other objects.
· Things do not move from their places because of the friction.
· Speeds of moving objects can be controlled due to friction only.
Friction is a foe because -
· It causes wear and tear in objects of our daily use including machines.
· It makes movement of heavy objects very difficult.
· We need to spend a lot of time and money for the maintenance of various machines in order to keep their friction low.
Question.10: Explain why objects moving in fluids must have special shapes.
Answer: The fluids like various liquids or air that is; gaseous medium also exerts frictional force known as Drag. To minimize the effect of this Drag, streamlining of the motion of the object in the fluid is necessary which can be achieved by modification in the shape of the body. Objects moving in fluids must have a special shape called streamlined shapes or aerodynamic shapes. Streamlined Shape or Aerodynamic Shape is that shape which overcomes the friction of fluid. They have pointed fronts with little broader middle portion with tapering at the back.
Class VIII Friction - further study