Notes On Friction and its Types - CBSE Class 8 Science
 Friction is a force that opposes the relative motion between two surfaces of objects in contact. The force of friction always acts in a direction opposite to that of the applied force. Friction exists between two surfaces due to irregularities on the surfaces of the objects in contact, interlocking of micro-level irregularities of the two surfaces and ploughing of harder surfaces into smoother surfaces. Factors That Affect Friction        •  Roughness of the surface        •  The extent to which the two surfaces press together        •  Nature of the surface Types of Friction There are three types of friction that exist between solid surfaces. They are static friction, sliding friction and rolling friction. Frictional force that comes into action before the start of the motion of an object is called static friction.  Example: a box placed on the floor. When an object slides over another surface, the frictional force that comes into action is sliding friction. Example:  a boy sliding on a slide. The frictional force exerted when an object rolls over a surface is called rolling friction. Example:  rolling ball on the ground. It is found that:  Rolling friction < sliding friction < static friction. The frictional force exerted by fluids when objects move through them is called fluid friction or drag.   Effects of Friction        •  Friction produces heat.        •  Friction causes wear and tear.        •  Friction opposes motion. Advantages of Friction        •  Friction between pen and paper enables us to write on the paper.        •  Friction between our feet and the ground allows our movements like standing, walking and running.        •  Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping. Disadvantages of Friction        •  Friction causes moving objects to stop or slow down.        •  Friction produces heat causing wastage of energy in machines.        •  Friction causes wear and tear of moving parts of macinery, soles of shoes, etc. Methods of Increasing and Reducing Friction In real life, there are circumstances where we have to increase the friction and minimise the friction. Friction can be increased by increasing the roughness of the surfaces in contact. For example, treading of shoes and tyres is done to increase friction. When friction is undesirable we have to reduce the friction. Friction can be minimised by using lubricants like oil and grease and by using ball bearing between machine parts. A substance that is introduced between two surfaces in contact, to reduce friction, is called a lubricant. Fluid friction can be minimised by giving suitable shapes to the objects moving in the fluids. Friction Is A Necessary Evil There are instances in daily life where friction is a necessity. For example, without friction, we cannot hold objects in our hands; we cannot walk and cannot light a match stick. Sometimes friction is not desirable, for example  friction between machinery parts, which causes wear and tear. As friction is advantageous to us it is considered as a friend but due to its disadvantages it is a foe. Depending on the circumstance, friction can be a help or a hindrance. Thus it is a necessary evil.

#### Summary

 Friction is a force that opposes the relative motion between two surfaces of objects in contact. The force of friction always acts in a direction opposite to that of the applied force. Friction exists between two surfaces due to irregularities on the surfaces of the objects in contact, interlocking of micro-level irregularities of the two surfaces and ploughing of harder surfaces into smoother surfaces. Factors That Affect Friction        •  Roughness of the surface        •  The extent to which the two surfaces press together        •  Nature of the surface Types of Friction There are three types of friction that exist between solid surfaces. They are static friction, sliding friction and rolling friction. Frictional force that comes into action before the start of the motion of an object is called static friction.  Example: a box placed on the floor. When an object slides over another surface, the frictional force that comes into action is sliding friction. Example:  a boy sliding on a slide. The frictional force exerted when an object rolls over a surface is called rolling friction. Example:  rolling ball on the ground. It is found that:  Rolling friction < sliding friction < static friction. The frictional force exerted by fluids when objects move through them is called fluid friction or drag.   Effects of Friction        •  Friction produces heat.        •  Friction causes wear and tear.        •  Friction opposes motion. Advantages of Friction        •  Friction between pen and paper enables us to write on the paper.        •  Friction between our feet and the ground allows our movements like standing, walking and running.        •  Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping. Disadvantages of Friction        •  Friction causes moving objects to stop or slow down.        •  Friction produces heat causing wastage of energy in machines.        •  Friction causes wear and tear of moving parts of macinery, soles of shoes, etc. Methods of Increasing and Reducing Friction In real life, there are circumstances where we have to increase the friction and minimise the friction. Friction can be increased by increasing the roughness of the surfaces in contact. For example, treading of shoes and tyres is done to increase friction. When friction is undesirable we have to reduce the friction. Friction can be minimised by using lubricants like oil and grease and by using ball bearing between machine parts. A substance that is introduced between two surfaces in contact, to reduce friction, is called a lubricant. Fluid friction can be minimised by giving suitable shapes to the objects moving in the fluids. Friction Is A Necessary Evil There are instances in daily life where friction is a necessity. For example, without friction, we cannot hold objects in our hands; we cannot walk and cannot light a match stick. Sometimes friction is not desirable, for example  friction between machinery parts, which causes wear and tear. As friction is advantageous to us it is considered as a friend but due to its disadvantages it is a foe. Depending on the circumstance, friction can be a help or a hindrance. Thus it is a necessary evil.

#### Activities

 Activity 1 Phet.colorado has created a beautiful simulation to depict that friction produces heat. When the yellow molecules are rubbed onto the green molecules a rise in temperature is observed. This indicates that friction produces heat. Go to Activity Activity 2  Learningcenter.nsta has created a simulation "Ball on a Flat Surface" which is designed to provide a surface with a selectable amount of friction. In this, the friction slider can be adjusted to set a value for friction and compare how far the ball travels in relationship to other values of friction. Go to Activity

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