Chapter: Friction : cls8
Chapter: 5 Friction
Friction:
A contact force that opposes or tends to oppose the flow of any Relative
motion.
The force is directed along the surfaces in contact, i.e. (tangential direction).
Basic Causes of friction:
Depends on two things.
The roughness of the surfaces in contact – More the roughness between them
that are in contact, the more the force of friction between them.
When two surfaces are put in contact, the irregularities of one surface gets
interlocked with the irregularities of another surface.
Factors affecting Friction:
a) Nature of the two surfaces in contact.
b) Force pressing the two surfaces together.
Static Friction
- When a force of friction, which balances the applied force during the
stationary state of a body.
- It is a self-adjusting force.
- It wants the objects to remain at rest and until an external force is applied.
- If static friction force equals the magnitude of the external force, it
starts moving once it exceeds the Limiting Force of Friction.
Limiting force of friction
- Maximum value of the force produced by Static friction when the
body is just sliding over the horizontal surface.
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Chapter: Friction : cls8
Sliding Friction
- The force of friction, between two surfaces when one of them is sliding
over the surface of the other.
- Relative motion starts when the interlockings get unlocked because the
Contact points don’t receive enough time to lock each other
- It is a little smaller than Static Friction.
- Thus, The Moving body is easily movable than a Stationary State of
Body
- Since the Moving body is already is in Motion hence their contact points
don’t get locked So It can be Easily “accelerated”.
- But in the stationary state of the Body It is difficult because
➢ Initial Velocity (u) = 0
➢ Contact particles are Locked.
Rolling Friction
- The force of friction, when one body rolls over the surface of another
body.
- Rolling friction is lesser than Sliding Friction since Less surface area is in
contact with the Other surface
- Least of All types of Friction.
So
Static Friction > Sliding Friction > Rolling Friction.
Invention and Significance of wheels:
- Rolling friction is less than sliding friction, which led man to invent
wheels.
- Wheel is considered one of the Greatest Inventions in the History of
Mankind.
- It makes us push11 a cart of Heavy load Easier.
- Wheels are used extensively in the Field of transportation.
- Labour and Energy is saved to a Great Extent.
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Chapter: Friction : cls8
Importance of Friction:
- Walking on the ground – Friction between the ground and our feet
enables us to walk.
- When we walk, we push the ground under our feet in the backward
direction; friction provides the forward movement.
- Water or oil, Present in the floor provides a thin layer between our feet
and floor. This decreases the friction between our feet and floor and
therefore, We tend to slip.
- Transportation - Friction between the tyres and road is necessary for
vehicles to move safely. If this friction is not present, there will be no
movement in the object.
- Day-to-day activities – Friction helps to stick the rough powder of
chalk on the surface of the blackboard. It also allows us to write with
Pencil and Pen which cannot be possible without Friction.
Friction – evil or nuisance:
1. Friction opposes relative motion between two objects in contact. Some
of the Applied efforts of the moving object is wasted.
2. It is responsible for lot of wear and tear of moving parts.
3. The significant amount of energy, supplied to a machine gets wasted in
the form of heat energy while overcoming the force of friction.
4. Causes wastage of energy and damages moving parts on machinery.
Methods to Increase friction.
1. The treads and grooves improve their grip of the vehicles on the road
which helps us to avoid skidding/slipping.
2. Sportsmen and Players use special types of sports shoes with
spikes/cleats on their soles to increase friction and Prevent Slipping.
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Chapter: Friction : cls8
3. Athletes and sportspersons make use of friction. Gymnasts apply the
coarse substance on hands to increase the friction between their hands
and uneven bars. Kabaddi players rub their hands with soil for having
better grip on their opponents.
Methods to Reduce Friction:
1. Polishing – Polishing decreases the roughness of the surface and
makes it smooth which decreases friction.
2. Lubrication – Oil like substances called Lubricants are added which
help to reduce the friction to increase efficiency. It can be liquids,
semi-liquids and solids.
3. Ball-bearings – Sliding friction is greater than rolling friction, Hence it is
replaced by Rolling Friction. So It is used in Shafts of Motors,
Dynamos, Axles and Many Machinery parts.
4. Air :-
- Compressed and Purified Air acts as a lubricant.
- Creates an elastic cushion between the moving parts, thus reducing
friction.
- Added Advantage of preventing dust and dirt from collecting on
the moving parts.
Fluid Friction:
Force of friction exerted by Fluids against a Moving body.
Dependence of force of fluid:
1. Nature of the fluids.
- Nature of the Fluid is related to the Viscosity of the Fluid.
- Viscosity of the Fluid ∝ Air Resistance
2. Shape of the moving object.
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- Air resistance ∝ Surface Area ∝
𝑆𝑝𝑒𝑒𝑑
(𝐼𝑛𝑑𝑖𝑟𝑒𝑐𝑡𝑙𝑦 𝑃𝑟𝑜𝑝𝑜𝑟𝑡𝑖𝑜𝑛𝑎𝑙)
3. Speed of the moving object.
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Chapter: Friction : cls8
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- Speed of the Moving Object ∝ 𝐹𝑙𝑢𝑖𝑑 𝐹𝑟𝑖𝑐𝑡𝑖𝑜𝑛.
Air resistance:-
Force of friction exerted by Air against a moving object.
Streamlined Body:-
- A Special shape which is designed that It is Narrow towards the front
(tip)and Broad at the middle.
- These Special-shaped bodies that reduce air resistance and help to
acquire fast speeds, are called Streamlined Bodies.
Advantages of Streamlined Shape
- Reduces Fluid Friction when the Body moves through Fluid.
- Minimizes the loss of Energy.
- Increases the Speed of the Moving Object.
Streamline flow/shape:
- Vehicles are designed that air smoothly flows over their surfaces so that
every air particle passes a particular point with same speed and
direction. This flow is called Streamlined Flow.
