LECTURE - 1 : CLASS VIII : SCIENCE : CHAPTER 12 : FRICTION

CLASS VIII   |    SCIENCE    |    CHAPTER 12

      Notes prepared by Subhankar Karmakar

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• FRICTION : 

The force which always opposes the motion of one object over another object in contact with it, is called friction. 

• friction occurs between the two surfaces which are in contact with each other. 

• friction is a force which occurs when the two objects tend to slide over each other and even when there are actually sliding over each other. 

• Direction of force of friction:

The force of friction always opposes the motion of one object over another object. The force of friction acts in a direction opposite to the direction in which an object moves. Therefore, the force of friction acts in a direction opposite to the direction of motion of an object.

• Cause of friction:

Friction is caused by the interlocking of irregularities in the surfaces of the two objects which are in contact with each other. 

When we try to move an object over the other, we have to apply a force to overcome interlocking of the irregularities in the their surfaces. More the roughness of a surface, larger is the number of irregularities on its surface and hence greater will be the friction. Thus, the force of friction is greater if very rough surfaces are involved. Friction will be less if the surface is smooth and polished. 

• Spring balance:

The spring balance is a device which is used for measuring force acting on an object. With the help of a spring balance we can measure the frictional force. 

The spring balance contains a coiled spring which gets stretched when a force is applied to its free end having a hook. The extent by which the spring gets structured is a measure of the force applied. Larger the stretching of spring, greater will be the magnitude of the force applied. the stretching of spring or magnitude of force is indicated by a pointer attached to the spring which moves on a graduated scale. The reading on the scale of spring balance give us the magnitude of force. When the spring balance is held vertically, it is used to measure the weight of an object hung from its hook. and when spring balance is held horizontally it can be used to measure the force being applied to pull the object on a horizontal surface.

• Factors affecting friction:

It has been found by experiment that friction between two surfaces depends on two factors.

• The nature of the two surfaces (smoothness or roughness of the two surfaces).
• The force with which two surfaces are pressed together.

The force of friction does not depend on the amount of surface area of the two objects which is in contact with each other. 

Dependence of friction on the nature of two surfaces:

• Friction depends on the smoothness or roughness of the two surfaces which are in contact with each other. 
• If the two contact surfaces are smooth, then the frictional force is less where as when the two contact surfaces are rough, frictional forces are more.