REAL LIFE EXAMPLES OF FRICTION

WHAT IS FRICTION?

REAL-LIFE EXAMPLES OF FRICTION

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 to it. Finally, it stops. Have 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?

Figure 12.1: A boy falls down when he steps on a banana peel

You will find the answers to such questions in this chapter.

Source: This topic is taken from NCERT TEXTBOOK

FORCE OF FRICTION

FORCE OF FRICTION

Activity 12.1:

*   Gently push a book on a table [Fig. 12.2(a)].

*   You observe that it stops after moving for some distance.

*   Repeat this activity pushing the book from the opposite direction [Fig. 12.2, (b)].

*   Does the book stop this time, too? Can you think of an explanation?

*   Can we say that a force must be acting on the book opposing its motion?

*   This force is called the force of friction.

Figure 12.2(a): 

Figure 12.2(b): Friction opposes relative motion between the surfaces of the book and the table

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.

Source: This topic is taken from NCERT TEXTBOOK

FACTORS AFFECTING FRICTION

FACTORS AFFECTING FRICTION

Activity 12.2:

*   Tie a string around a brick. Pull the brick by a spring balance (Fig. 12.3).

*   You need to apply some force. Note down the reading on the spring balance when the brick just begins to move.

*   It gives you a measure of the force of friction between the surface of the brick and the floor.

*   Now wrap a piece of polythene around the brick and repeat the activity.

*   Do you observe any difference in the readings of the spring balance in the above two cases?

*   What might be the reason for this difference?

*   Repeat this activity by wrapping a piece of jute bag around the brick.

*   What do you observe?

Figure 12.3: A brick is being pulled by a spring balance

Activity 12.3:

*   Make an inclined plane on a smooth floor, or on a table.

*   You may use a wooden board supported by bricks, or books. [Fig. 12.4 (a)].

*   Put a mark with a pen at any point A on the inclined plane.

*   Now let a pencil cell move down from this point.

*   How far does it move on the table before coming to rest?

*   Note down the distance. Now spread a piece of cloth over the table.

*   Make sure that there are no wrinkles in the cloth.

*   Try the activity again [Fig. 12.4 (b)].

*   Repeat this activity by spreading a thin layer of sand over the table.

*   Maintain the same slope throughout the activity.

Figure 12.4(a): 

Figure 12.4(b): The pencil cell covers different distances on different surfaces

In which case is the distance covered the minimum? Why is the distance covered by the pencil cell different every time. Try to reason why? Discuss the result.

Does the distance cover 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.

Figure 12.5: Surface irregularities

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 person is sitting on it.

Figure 12.6: You have to push on the box to keep it moving

Recall your experience when last time you moved a heavy box from one place to another (Fig. 12.6). 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.

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 static friction and you find it somewhat easier to move the box already in motion than to get it started.

Spring Balance

Spring balance is a device used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on  a graduated scale. The reading on the scale gives the magnitude of the force.

Source: This topic is taken from NCERT TEXTBOOK