NCERT Class VIII Science Solutions: Chapter 12-Friction Part 2

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Question 6:

Explain why sportsmen use shoes with spikes.

Answer:

Spikes increase friction and give better grip with the ground. It avoids slipping of sportsmen while playing or running. Sportsmen use shoes with spikes because these shoes give them a better grip while running. This is because the force of friction between the shoes and the ground increases with the help of spikes.

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:

Friction force is directly proportional to mass. Heavier box will put more force on the floor surface as compared to lighter box and therefore will experience more opposite force (friction). Seema will have to apply larger force.

Question 8:

Explain why the sliding friction is less than the static friction.

Answer:

When the objects are at rest, the interlocking of irregularities in the two surfaces of the objects is higher than that of when objects are moving. When objects are moving, there is less interaction between their surfaces. That's why sliding friction is less than static friction.

Question 9:

Give examples to show that friction is both a friend and a foe.

Answer:

Friction is both a friend and a foe due to following reasons:

Friction act as friend:

  • We are able to walk because of frictional forces between ground and our feet.

  • We are able to write because of friction between ball point and the paper surface.

  • Nails and screws stick to wall surfaces because of friction.

  • Lighting a match stick is because of friction between match stick and the side surface of match box.

Friction act as foe:

  • Wear and tear of soles of our shoes is due to friction.

  • When a tire deflates, it is difficult to move the vehicle because of increased friction between the tire and road surface.

  • Machines gets heated up because of friction.

  • Due to friction, machines and vehicles consumes more fuel and increase maintenance cost.

Question 10:

Explain why objects moving in fluids must have special shapes.

Answer:

The frictional force exerted by fluids is also called drag or fluid friction. To overcome or minimize air fluid friction, the bodies of objects minimize its surface i.e. sometime spherical shapes or other streamlined shape with smooth surface. Aero planes, jets, rain drops have streamlined curve surface to reduce air drag

Result for images objects moving in fluids must have special …

Objects Moving in Fluids Must Have Special Shapes

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