NCERT Book Class 8 Science Friction

FRIICTIION

You might have seen a driver of a car or a truck slowing down the vehicle at a traffic signal. You, too, slow down your bicycle whenever needed by applying brakes. Have you ever thought why a vehicle slows down when brakes are applied? Not only vehicles, any object, moving over the surface of another object slows down when no external force is applied on it. Finally it stops. Hav you not seen a moving ball on the ground stopping after some time? Why do we slip when we step on a banana peel (Fig. 12.1)? Why is it difficult to walk on a smooth and wet floor? You saw that if you apply the force along the left, friction acts along the right. If you apply the force along the right, the friction acts along the left direction. In both cases the force opposes the motion of the book. The force of friction always opposes the applied force. In the above activity, the force of friction acts between the surface of the book and the surface of the table. Is the friction the same for all the surfaces? Does it depend on the smoothness of the surfaces? Let us find out.

12.2 Factors affecting

FrictionDoes the distance covered depend on the nature of the surface on which the cell moves? Could the smoothness of the surface of the cell also affect the distance travelled by it? Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them (Fig. 12.5). Irregularities on the two surfaces lock into one another. When we attempt to move any surface, we have to apply a force to overcome interlocking. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved.

We see that the friction is caused by the interlocking of irregularities in the two surfaces. It is obvious that the force of friction will increase if the two surfaces are pressed harder. You can experience it by dragging a mat when nobody is sitting on it, and when a preson is sitting on it.

Recall your experience when last time you moved a heavy box from one place to another. If you have no such experience, get that experience now. What is easier — to move the box from rest, or to move it when it is already in motion? 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. friction and you find it somewhat easier to move the box already in motion than to get it started.

12.3 Friction : A Necessary Evil

Recall now some of your experiences. Is it easier to hold a kulhar (earthen pot) or a glass tumbler? Suppose the outer surface of the tumbler is greasy, or has a film of cooking oil on it; would it become easier or more difficult to hold it? Just think : would it be possible to hold the glass at all if there is no friction?

Recall also how difficult it is to move on a wet muddy track, or wet marble floor. Can you imagine being able to walk at all if there were no friction? When the box 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 staticYou could not write with pen or pencil if there were no friction. When your teacher is writing with chalk on the blackboard, its rough surface rubs off some chalk particles which stick tothe black board. Could it happen if there were no friction between the chalk and the board?

If an object started moving, it would never stop if there were no friction. Had there been no friction between the tyres of the automobiles and the road, they could not be started or stoped or turned to change the direction of motion. You could not fix a nail in the wall (Fig. 12.7) or tie a knot. Without friction no building could be constructed.

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.

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

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

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.

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?

6. Explain why sportsmen use shoes with spikes.

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?

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

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

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

Please refer to attached file for NCERT Class 8 Science Friction