Class 11 Physics practical reading To study the relationship between force of limiting friction and normal reaction and to find the coefficient friction between a block and horizontal surface.

Aim

To study the relationship between the force of limiting friction and normal reaction and to find the coefficient friction between a block and horizontal surface.

Apparatus Required

A horizontal plane with a frictionless pulley at its one end, a wooden block with a hook, a spring balance (0 - 500 gm), a spirit level, a scale pan and a fine thread.

Theory

Friction is the opposing force when an object moves over a surface. Its value depends upon the nature of the surfaces in contact. It is of two types: (i) static friction and (ii) kinetic friction. 

Static friction comes into play between the surfaces of the two objects before one starts moving over the other. The maximum value of static friction is called limiting friction. Kinetic or dynamic friction between the two surfaces when one surface is in steady motion over other surfaces. It is always less than the limiting friction.

When a block of weight W is placed on a horizontal surface, weight and normal reaction act in opposite directions as shown in Fig. When the block is pulled by a force F, it is balanced by a force of friction f between the block and surface. In a balanced position, 

Weight W = Normal Reaction N and Applied force F = Force of friction f When a force is applied on a block to just move it from rest, the force of limiting friction comes into play. 

This limiting frictional force F1 is given by    F1 = μ1N 

or the coefficient of friction

                   μ1 = F1 / N

Procedure

1. Put the horizontal plane on. the table top and level it with the help of spirit level. 

2. Weight the wooden block using a spring balance and tie one end of the thread with its hook and pass it over the pulley. 

3. Find the weight of the pan and tie other free end of the thread with it. 

4. Put some weights on the pan from the weight box and tap the table top gently using a pencil and adjust the weight in pan gradually so that the block just starts sliding on tapping the table top. 

5. Note the total weights put in the pan and record it in the observation table. 

6. Next put a 50 gm weight over the wooden block and repeat above steps till the block just starts moving. 

7. Repeat above steps 4 to 6 five more time and record the observations.

Observations

Precautions

1. Pulley should be frictionless. 

2. Table top should be horizontal. 

3. Table top should be tapped gently.
4. The pan should not touch the table.

Sources of Error

1. Table top may not be horizontal. 

2. Pulley may not be frictionless. 

3. Thread may not be horizontal between block and pulley.