CBSE Class 8 Answered

(a) Sliding friction

 

Explanation: Friction is the force which resists the relative motion between two surfaces in contact.

Now, rolling friction comes into play between two surfaces, when one body rolls over the other and sliding friction comes into play when when one body slides or tends to slide over the surface of another body; it tends to reduce the relative velocity between the two surfaces during sliding. Now, if the area of contact between the two surfaces can somehow be reduced, then the effect of friction between the two surfaces will be drastically reduced. 

When an object slides across the ground, it has much more surface area in direct contact with the ground, which means that the amount of friction is significantly higher. When an object rolls along the ground, only a minuscule point on the object contacts the ground at any point in time, making the stopping force much weaker.

Moreover, in sliding motion, the heights of cavities (irregularities) are to climb up which introduces the friction, while rolling friction, the heights of cavitites are to cross over which avoid the friction.