Chapter 12 Friction

By | June 3, 2018
Class 8 | Science | Ch 12 |Friction| NCERT Solutions

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 carrom board ________ friction.
(e) Sliding friction is ___________ than the static friction.

Answer:
(a) Friction opposes the motion between the surfaces in contact

with each other.
(b) Friction depends on the nature of surfaces.
(c) Friction produces heat.
(d) Sprinkling of powder on the carrom board reduces friction.
(e) Sliding friction is less than the static friction.

2. Four children were asked to arrange forces due to rolling, static and sliding
frictions in a decreasing order. Their arrangements are given below.
Choose the correct arrangement.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling

Answer: (c) static, sliding, rolling

3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper
and towel spread on the floor. The force of friction acting on the car on
different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper

Answer: (a) wet marble floor, dry marble floor, newspaper and towel.

4. Suppose your writing desk is tilted a little. A book kept on it starts sliding
down. Show the direction of frictional force acting on it.

Book moving downwards

When book slides down on the desk, a frictional force acts between the book and the surface of the desk. The direction of the friction force on the book is opposite to the direction of its motion and acts in an upward direction.

5. You spill a bucket of soapy water on a marble floor accidently. Would it
make it easier or more difficult for you to walk on the floor? Why?

It becomes difficult to walk on the soapy floor.For walking, we push the ground in a backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk.Walking on slippery ground is difficult because the frictional force on slippery ground is much less which may not be sufficient to prevent us from slipping.

6. Explain why sportsmen use shoes with spikes.

Spikes are the pointed nails which get into the ground and increase friction between shoe and the ground.The players and athletes have to run fast. Greater friction is required between the soles of their shoes and ground to prevent slipping. To increase friction, Spikes are provided in the soles of shoes worn by players and athletes.This prevents the slipping of player or athlete on running.

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?

Friction force is directly proportional to mass.Force of friction arises because of interlocking of irregularities on the two surface in contact.When a heavy object is placed, the interlocking of irregularities on the surface of box and floor become strong because two surfaces in contact are pressed harder.Hence more force is required to overcome the interlocking.Thus, to push heavier box, Seema has to apply a greater force than Iqbal.

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

The maximum frictional force present between any two objects when one object just tends to move or slip over the surface of the other object ,is called static friction.

The frictional force present when one object moves slowly or slides over the surface of another object is known as sliding friction.

When irregularities present in the surfaces of two objects in contact get interlocked with each other, friction came into play.  The time given in sliding for interlocking is very small. Thus, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Due to this reason, sliding friction is less than static friction.

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

Advantages of Friction

1) Friction enables us to walk without slipping

2) Friction enables a car to move on road without skidding

3) Friction enables us to apply brakes and slow down or stop a moving car

4)Friction enables us to write and draw on paper

5)Friction enables us to pick up and hold things in our hands

6) Nails can be fixed in a wall due to friction

7) Friction enables us to light a matchstick

Disadvantages of friction

1)Friction wears away the soles of our shoes

2)The tyres of vehicles wear out gradually due to friction

3)Friction wears out the rubbing machine parts

4)Friction wears out the brake pads of vehicles gradually

5)Friction produces heat which may damage machines

6)Friction reduces the efficiency of Machines

7)Friction slows down motion

10. Explain why objects moving in fluids must have special shapes.

Water exerts force of friction on object which move through them and opposite their motion.When an object moves through water, it pushes the water out of the way and the water pushes back on the object.This push of water on the moving object creates friction which tends to slow down the moving object.Drag is a kind of frictional force exerted by fluid which opposes the motion of an object through that fluid.Drag force acts in direction opposite to the direction of motion of the object.A body shape which offers very little resistance to the flow of air or water around it is called streamlined shape.The fluid friction or drag can be reduced or minimised by giving special shape called streamlined shape to the objects which move through fluids.When an object having a streamlined body shape moves very fast, then the fluid can flow past the moving objects smoothly, reducing the fluid friction or drag.

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