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 of force of friction between the surface of the brick and the floor.
Answers
Fig. 12.2 (a), (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.
12.2 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 of force of friction between the surface of the brick and the floor.
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.
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 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.
Answer:
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.
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