NCERT Class 8 Science Chapter 12 Solution
Brief Information: This post contains the solution for Chapter 12 Friction of Class 8 of the CBSE and also Gujarat Board (GSEB) books by GCERT (based on new NCERT).
Q1. Fill in the blanks.
(a) Friction opposes the relative motion between the surfaces in contact with each other.
(b) Friction depends on the smoothness of surfaces.
(c) Friction produces heat.
(d) The sprinkling of powder on the carrom board reduces friction.
(e) Sliding friction is less than the static friction.
Q.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
Ans. (c) static, sliding, rolling.
Q.3. Alida runs her toy car on a 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.
Ans. (a) wet marble floor, dry marble floor, newspaper and towel.
Q.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.
Ans. The Frictional force will act upward. It will be in opposite direction to sliding motion of the book.
Q.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?
Ans. Soaps are used as surfactant. They reduce the friction between two surface. The soapy water on the floor will reduce the friction bewteen floor and feet. The foot will not be able get sufficient grip. It makes surface slippery. The person walking on such surface will face difficulty and he may slip.
Q.6. Explain why sportsmen use shoes with spikes.
Ans. The spikes on the shoes increase the friction between the foot and the ground. The sportsmen will not slip due to high frictional force and it will help to move forward with ease.
Q.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?
Ans. The heavy box has higher frictional force compared to lighter box. The heavy box is pressed tightly downwards which produce more friction. Due to this Seema will need to apply larger force than Iqbal.
Q.8. Explain why sliding friction is less than static friction.
Ans. The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction. When the object starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So, the sliding friction is slightly smaller than the static friction and you find it somewhat easier to move the object already in motion than to get it started.
Q.9. Give examples to show that friction is both a friend and a foe.
Ans. Friction is the force resisting the relative motion of object on surface or through fluids. Friction is both friend and a foe, it is clear from the below given example.
- Friction as a friend:
- It provides grip to hold object in hand.
- It helps us to walk comfortably on the floor.
- Applying brakes in car or vehicle enables to reduce speed or stop it.
- Writing on paper with pen or on board with chalk is possible due to frictio
- It provides grip to hold object in hand.
- Friction as a foe:
- It causes wear and tears in objects such as machines and tools which further require money to get them repaired.
- It reduces the speed of moving objects, so more force is required.
- It produces hurdles in moving any object freely.
- It produces heat e.g jar or mixer becomes hot after running for few minutes
Q.10. Explain why objects moving in fluids must have special shapes.
Ans. 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.