NCERT Solutions for Class 8 Science Chapter 12 Friction

Ans : (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  
(e) Sliding friction is  less  than the static friction. 

Ans : (c) static, sliding, rolling  
Friction comes into play when irregularities present in the surfaces of two objects in contact get interlocked with each other. Static friction comes into play when we try to move an object which is at rest. Sliding friction comes into play when an object slides over the surface of another object. In sliding friction, the time given for interlocking is very small. Hence, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Because of this reason, sliding  friction is less than static friction. Similarly, the area of contact in case of rolling friction is smallest as compared to static or sliding friction. This area of contact changes gradually because of rolling. Hence, rolling friction is lesser than both static and sliding friction. Thus, the correct sequence is—static, sliding, rolling. 

Ans : 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 

Ans : When a book slides on the writing desk, a frictional force acts between the book and the surface of the desk. The direction of frictional force on the book is opposite to the direction of its motion and acts in upward direction, as shown in the following figure. 
 

Ans : We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction between the ground and feet decreases when there is soapy water spilled on the floor. Hence, it becomes difficult to walk on the soapy floor.