Friction Class 8
Topics To Be Covered
(i) Introduction of friction
(ii) Cause of friction
(iii) Factor effecting friction
(a) Dependence of friction on the nature of two surfaces.
(b) Dependence of friction on the force with which two surfaces are passed together.
(iv) Types of friction
(a) Static friction
(b) Sliding friction
(c) Rolling friction
(v) Advantage of friction
(vi) Disadvantage of friction
(vii) Method of increasing friction
(viii) Method of reducing friction
(ix) Fluid friction: Friction exerted by liquid and gas
(x) Factors affecting the fluid friction
(xi) Disadvantage of fluid friction
(xii) Method of reducing fluid friction
• Friction →
The force which always opposes the motion of one object over another object in contact with it is called friction.
Direction of force of friction →
The force of friction acts in a direction opposite to the direction in which an object moves.
Or,
The frictional force or force of friction acts in the opposite direction of the motion of the object.
Note → The force of friction opposes the relative motion between the surfaces of two moving objects.
Cause of friction:
Friction is caused by the interlocking of irregularities in the surfaces of two objects which are in contact with each other.
• Factors effecting friction
It has been formed by experiments that the friction between two surfaces depend on two friction.
(i) The nature of the surfaces (smoothness and roughness of two surfaces)
(ii) The force with two surfaces are passed together.
(a) Dependence of friction on the nature of two surfaces
Friction is not same for all surfaces.
Friction depends on the smoothness or roughness of two surfaces which are contact with each other.
When two surfaces in contact are smooth then the friction between them will be small because the interlocking is two smoothness.
(b) Dependence of friction on the force with which two surface are passed together.
Friction is caused by interlocking of irregularities of the two surfaces when one object is placed over another object.
⇰ If the two surfaces of objects are pressed together harder by a greater force then the friction will increase because pressing together two surfaces of objects with a greater force will increase the interlocking in the two surfaces.
- Types of Friction
(i) Static friction
(ii) Sliding friction
(iii) Rolling friction
(i) Static friction → The maximum frictional force present between any two objects when one object just tends to move or slip over the surface of the other object is called static friction.
⇰ Static friction is the kind starting friction because an object just tends to start moving, it does not actually move.
(ii) Sliding friction → The frictional force present when one object moves slowly over the surface of another object is known as sliding friction.
⇰ Thus the sliding friction comes into play when an object is sliding (moving slowly but continuously) over the another object.
(iii) Rolling friction → When an object like a wheel rolls over the surface of another object, the resistance to its motion is called rolling friction.
⇰ It is always easier to roll than to slide an object over another object.
⇰ Rolling friction is much lesser than the sliding friction hence rolling reduce the friction
Q. Question based on sliding friction
Q. Why sliding friction is less than static friction.
Ans → When an object has already started moving or sliding, the irregularities on its surface do not get enough time to lock into the irregularities on the surface of the other object completely.
Hence the interlocking of the two surfaces is less when an object has already started moving. The sliding friction is smaller than the static friction.
Advantage of friction
(i) Friction enables us to walk without sleeping.
(ii) Friction enables a car to move on road without skidding.
(iii) Friction enables to apply break and slow down or stop a moving car.
(iv) Friction enables us to write and draw on paper.
(v) Friction enables us to pick up and hold things in our hand.
(vi) Nails can be fixed in a wall or wood due to friction.
(vii) Friction enables us to light a matchstick.
(viii) Friction enables us to cut wood with a saw.
Disadvantage of friction
(i) Friction wears away the soles of our shoes.
(ii) The tires of vehicles wear out gradually due to friction.
(iii) Friction wear out the rubbing machine part.
(iv) Friction wear out the break pad of vehicles gradually.
(v) Friction wear out the steps of staircases in building and foot over bridge.
(vi) Friction produces heat which may damage machines.
(vii) Friction reduces the efficiency of machine.
(viii) Friction slows down motion.
Method of increasing friction
(i) A long and shallow cut or depression in the surface of a hard material is called groove. A series of patterns made into the surface of a tire (to provide grip on the road) is called tread.
⇰ As we know that friction is very useful to us, it can be increased by making the surface of an object rough.
There are many ways to increase the friction
(i) Grooves are made in the souls of shoes to increase the friction and prevent slipping.
(ii) Trads are made in the tire of vehicle to increase the friction and prevent skidding of vehicles on wet roads.
(iii) Spikes are provided in the shoes of players and athlete’s to increase the friction and prevent slipping.
(iv) Gymnasts apply some coarse substance on their hands to increase friction for better grip.
(v) Machine belts are made up special material to increase friction and drive machine wheels properly.
✶ Method of Reducing the friction
Some friction is harmful to us that’s the reason we reduce friction so as to make it less harmful to us.
Example : friction between moving the parts of machine causes a lot of wear and tear to the machine parts.
⇰ Friction can be reduced by making the following methods.
(i) Friction can be reduced by making the surface smooth by polishing.
(ii) Friction can be reduced by applying lubricant (like oil or grease) to the rubbing surface.
(iii) Friction can be reduced by using wheels to move objects.
(vi) Friction can be reduced by using ball bearings between the moving parts of machine.
✶ Fluid friction :
Friction exerted by liquid and gas.
✦ Those substance which are able to flow easily are called fluid.
✦ Fluids have no fixed shape.
✦ Liquids and gases are fluids (because they can flow easily).
✦ Water is the most common liquid around us.
✦ The most common gas around us is air.
✦ There is a friction whenever an object moves through a fluid, is called fluid friction.
✶ The frictional force exerted by a fluid (air or water) is called drag.
⇰ Drag acts in a direction opposite to the direction of motion of the object.
✶ Factors affecting the fluid friction
✦ The magnitude of frictional force or drag exerted by a fluid on an object moving through it depends on four factors :
(i) speed of the object
(ii) shape of the object
(iii) size of the object
(iv) nature of the fluid or viscosity of the fluid
(i) Speed of the object →
Higher the speed of an object moving through a fluid, greater will be the frictional force.
(ii) Shape of the object →
Object having streamlined shapes face much less frictional force. When moving through the a fluid, then the object which do not have streamlined shapes.
(ii) Size of the object →
Larger the size of an object moving through a fluid, greater will be the frictional force.
(iii) Nature of the fluid or viscosity of the fluid →
Higher the viscosity or thickness of fluid, greater will be a frictional force, acting on an object moving through it.
Disadvantage of fluid friction
(i) Fluid friction reduces the speed of object moving through the fluid.
(ii) When object moves through the fluids (air or water), they lose some of their energy in overcoming the fluid friction.
Method of reducing fluid friction
(i) The fluid friction or drag can be reduced or minimized by giving special shape called streamlined shape to the objects which moves through fluids.
For example → An airplane has a streamlined shape to reduce air friction that it encounters when flying at high speed through the sky.
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