Chapter 8 . FRICTION

Exercises :

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 or roughness of surfaces.

(c) Friction produces heat .

(d) Sprinkling of powder on the carrom board reduces friction.

(e) Sliding friction is usually 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 : The correct arrangement is (a) rolling, static, sliding.

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 : The correct arrangement in increasing order is (c) towel, newspaper, dry marble floor, wet marble floor.

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.

Answer : The frictional force acting on the book will be in the opposite direction to its motion. In this case, as the book is sliding down, the frictional force will act upward, opposing the motion of the book.

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?

Answer : Spilling a bucket of soapy water on a marble floor would make it more difficult to walk on the floor. The soap reduces the friction between the soles of your shoes and the floor, resulting in less traction and making it easier to slip or lose balance while walking.

6. Explain why sportsmen use shoes with spikes.

Answer :  The spikes on the shoes provide additional points of contact with the surface they are playing on, such as a track or a field. These spikes dig into the ground, increasing friction between the shoe and the surface. As a result, sportsmen can push off the ground more effectively, maintain better balance, and achieve faster speeds or better stability during their movements.

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 a larger force to push the heavier box. This is because the force required to move an object is directly proportional to its mass. The heavier box has more mass, requiring a greater force to overcome its inertia and initiate motion.

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

Answer : Sliding friction is less than static friction because it takes more force to overcome the initial resistance of an object at rest (static friction) compared to the force required to keep an object in motion (sliding friction).

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

Answer : Examples of friction as a friend:

(i). Friction between the soles of our shoes and the ground helps us walk or run, providing traction and preventing slipping.

(ii). Friction between the brake pads and the wheels of a vehicle allows us to stop or slow down the vehicle.

Examples of friction as a foe:

(i). Friction between moving parts of machinery causes wear and tear, leading to reduced efficiency and the need for maintenance.

(ii). Friction between the tires and the road can decrease fuel efficiency in vehicles, resulting in higher energy consumption and increased costs.

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

Answer : The frictional force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid. when objects move

through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimise friction. So, objects are given special shapes.