# CLASS 8 SCIENCE CHAPTER-12 FRICTION

Question 1. Fill in the blanks.
(a) Friction opposes the ______________ between the surfaces in contact with each other.
(b) Friction depends on the ______________ surfaces.
(c) Friction produces ______________.
(d) Sprinkling of powder on the carom board__________ friction.
(e) Sliding friction is ____________ than the static friction.

Solution :
(a) Friction opposes the relative motion between the surfaces in contact with each other.
(b) Friction depends on the nature surfaces.
(c) Friction produces heat.
(d) Sprinkling of powder on the carom board reduce friction.
(e) Sliding friction is less than the static friction.

Question 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
Solution :
(a) Static, Sliding, Rolling
When the irregularities present in the surfaces of two objects in contact get interlocked with each other, friction comes into play. When we try to move an object which is at rest, static friction comes into play. When an object slides over the surface of another object, sliding friction comes into play. In this friction, the time given for interlocking is very small. Hence, interlocking is not strong. Therefore, a lesser force is required to overcome this interlocking. And 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. Because of rolling, the area of contact changes gradually. Hence, rolling friction is lesser than both static and sliding friction. Thus, the correct sequence of frictions is static, sliding and rolling.

Question 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
Solution : (a) Wet marble floor, dry marble floor, newspaper and towel.

Question 4.Suppose your writing desk it tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Solution : When a book slides down on the tilted 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.

Question 5. If a bucket of soapy water is spilled on the marble floor accidently, is it easier or difficult for a person to walk on the floor?
Solution : It is easier to walk on the floor because of the friction present between our feet and the ground. For walking, we push the ground in a backward direction 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 spilled on the floor. Hence, it becomes difficult to walk on the soapy floor.

Question 6. Why do sportsmen use shoes with spikes?
Solution : Sportsmen use shoes with spikes because of the better grip given by spikes while running. This is because the force of friction between the shoes and the ground increases with the help of spikes.

Question 7. Rahul has to push a box and Rashmi has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Solution : Due to the interlocking of the irregularities on the two surfaces in contact, the force of friction arises. On the floor when a heavy object is placed, the interlocking of irregularities on the surface of box and floor become strong. This is because the two surfaces in contact are pressed harder. Hence, more forced is required to overcome the interlocking. Thus, to push the heavier box, Rashmi has to apply greater force than Rahul.

Question 8. Explain why the sliding friction is less than the static friction.
Solution : Friction is basically the irregularities present between two objects in contact which gets interlocked with each other. In case of sliding, there is not much time available for objects to get interlocked. Hence, less force is required to overcome this friction. Thus, sliding friction is less than static friction.

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

Solution :
Following are some examples that show friction is friend:
(i) We are able to write because of friction between ball point of pen and the paper surface.
(ii) We are able to walk because of friction.
(iii) We are able to write with chalk on blackboard because of friction.
(iv) We are able to drive the automobile because of friction between the tyres and the road.
(v) We are able to fix a nail in the wall because of friction.
Following are some examples that show friction is foe:
(i) Soles of shoes and tyres wear out because of friction.
(ii) Friction wears out the materials whether they are screws or ball bearings.
(iii) Steps of foot over-bridges at railway stations get worn-out because of friction.
(iv) Machines get heated up because of friction.
(v) When a tyre deflates, it is difficult to move the vehicle because of increased friction between the tyre and road surface.

Question 10. Explain why objects moving in fluids must have special shapes.
Solution : Whenever any object moves through a fluid, friction force tries to oppose its motion through the fluid. This opposing force is known as the drag force. To reduce this force, the bodies of objects needs to minimise its surface i.e. sometime spherical shapes or other streamlined shape with smooth surface. Example: Fishes have streamlined bodies to reduce the water drag.

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