NCERT Solutions for Class 8 Science Chapter 9 Friction(updated Pattern)
The frictional force exerted by fluids is also called drag or fluid friction. To overcome or minimize air fluid friction, the bodies of objects minimise their surface i.e. sometimes spherical shapes or another streamlined shape with smooth surfaces. Aeroplanes, jets, and drops have streamlined curve surfaces to reduce air drag.
Exercise Questions
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.
Solution:
(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 lesser than the static friction.
SOIL POLLUTION: CAUSES, IMPACTS, AND EFFECTIVE SOLUTIONS FOR A HEALTHIER ENVIRONMENT
2. Four children were asked to arrange forces due to rolling, static and sliding frictions in 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
Solution:
The answer is (c) static, sliding, rolling
3. Alida runs her toy car on the 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
Solution:
The answer is (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 the frictional force acting on it.
Solution:
The frictional force will act upward, i.e., in the direction opposite to that of the sliding book.
5. You spill a bucket of soapy water on a marble floor accidentally. Would walking on the floor make it easier or more difficult for you? Why?
Solution:
It is possible to walk on the floor because of the friction present between our feet and the ground. For walking, we push the ground backwards with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction decreases between the ground and the feet when there is soapy water spilt on the floor. Hence, it becomes difficult to walk on the soapy floor.
6. Explain why sportsmen use shoes with spikes.
Solution:
Spikes present on the shoes make the surface of the shoes rough. This increases the friction. As friction increases, the grip on the ground increases. This makes it easier for them to walk or run.
NCERT Solutions for Class 8 Science Chapter 7 Force and Pressure
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?
Soln:
A heavy object produces more friction as it is pressed hard against the opposite surface. So Seema will have to apply a larger force.
8. Explain why sliding friction is less than static friction.
Solution:
When irregularities present in the surfaces of two objects in contact get interlocked with each other, friction comes 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.
Solution
Advantages of friction
a) Due to friction, we can walk.
b) We can write because of the friction between the tip of the pen and the paper.
Disadvantages of friction
a) Because of friction, the tires and soles of shoes wear out.
b) Friction produces heat between different parts of the machines. This can damage the machines.
10. Explain why objects moving in fluids must have special shapes.
Solution:
When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. This opposing force is known as the drag force, and the frictional force depends on the shape of the body. By giving the objects a special shape, the force of friction acting on them can be minimised. Hence, it becomes easier for a body to move through the fluid.
