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Jaydeep Patel

School of Technology,

FrictionFrictionFrictionFriction

� When a body moves over another body, its motion is opposed by a resistingforce along the surfaces of contact.This resisting force is called Friction.

� Magnitude of this frictional (resisting) force depends mainlyupon the materials of the two bodies, upon the quality of the surfaceroughness and upon the pressure between the two surfaces of contact.

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� Force of the friction will be zero for 100% smooth or perfectlysmooth surfaces.

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smooth surfaces.

� Practically it is impossible to make ideal or 100% smoothsurfaces.

� Best possible smooth surfaces do have minute irregularities onthe surfaces and hence force of friction will be negligible butwill not be zero

Definition Of Friction ForceDefinition Of Friction ForceDefinition Of Friction ForceDefinition Of Friction Force

� When a motion occurs or about to occur between the two surfaces incontact, at that time because of interlocking of irregularities these surfacesoppose the motion.

� Resisting force, generated due to interlocking, opposite to themotion is calledThe Force of Friction.

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� If contact surface is smooth. friction force (F) will be less.

� If contact surface is rough, friction force (F) will be more.

� In machine parts like piston, bearings, liner etc. attempts are made toreduce friction to increase life of parts and efficiency of machine.

� Outer surface of vehicle tyres is made rough to increase frictionbetween tyre and road surface for safe driving.

� High friction (lots of friction) – will slow something down

� Low friction (not much friction) – will keep things moving

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High friction or Low friction?High friction or Low friction?High friction or Low friction?High friction or Low friction?

Ski’s on the snow

Car tyre

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Brakes on a bike Water on a slide

Pencil and rubber

Limiting FrictionLimiting FrictionLimiting FrictionLimiting Friction� Consider a body resting on the surface. If a little force is applied on the body, it

will not move, because friction force (F) will resist the motion.

� With further increase in external force, body will not move, But there is a limit ofdeveloping friction force.

� If external force becomes greater than that friction force body will move.

� The maximum friction force that can be developed at the contact

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� The maximum friction force that can be developed at the contactsurface, when body is just on the point of moving is called limitingforce of friction.

Types of Types of Types of Types of FrictionFrictionFrictionFriction

Friction

Static Friction

Dynamic Friction

Sliding Friction

Rolling Friction

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� Static friction :

� Friction experienced by a body when it is at rest is called static friction.

� In case of static friction,

if (Applied force < Friction force) then there is no Motion.

� Dynamic friction :

� Friction experienced by a body, when it is in motion is called dynamic friction.

� Dynamic friction is always less than static friction.

� Sliding friction :� Sliding friction :

� Friction experienced by a body when it slides over another body, is called slidingfriction.

� e.g. to push the table on floor.

� Rolling friction :

� Friction experienced by a body when it rolls over another body is called rollingfriction.

� e.g. - to move the drum by rolling, Ball bearing used in machines.5 - 9

A Flower pot on top of a tablestriking a match against a rough surface

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Rolling friction slows down the motion of a tire rolling along the pavement

Angle of FrictionAngle of FrictionAngle of FrictionAngle of Friction

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Coefficient Of FrictionCoefficient Of FrictionCoefficient Of FrictionCoefficient Of Friction

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Angle Of ReposeAngle Of ReposeAngle Of ReposeAngle Of Repose

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Variation Of Friction (F) With Applied Force (P)Variation Of Friction (F) With Applied Force (P)Variation Of Friction (F) With Applied Force (P)Variation Of Friction (F) With Applied Force (P)

� If the force P is increased, the friction force F alsoincreases, continuing to oppose P, until its magnitudereaches a certain maximum value Fm.

� If P is further increased, the friction force cannot balance itany more and the block starts sliding.

� As soon as the block has been set in motion, the

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� As soon as the block has been set in motion, themagnitude of F drops from Fm to a lower value Fk.

� This is because there is less interpenetration between theirregularities of the surfaces in contact when thesesurfaces move with respect to each other.

� From then on, the block keeps sliding with increasingvelocity while the friction force, denoted by Fk calledthe kinetic-friction force, remains approximately constant.

Laws of static frictionLaws of static frictionLaws of static frictionLaws of static friction

� Following are the laws of static friction.� The friction force always act in a direction, opposite to that in which the body

tends to move.

� The magnitude of friction force is equal to the external force.

F = P

� The ratio of limiting friction (F) and normal reaction (N) is constant.The ratio of limiting friction (F) and normal reaction (N) is constant.

F/N =µ

� The friction force does not depends upon the area of contact between the twosurfaces.

� The friction force depends upon the roughness of the surfaces.

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Laws of dynamic frictionLaws of dynamic frictionLaws of dynamic frictionLaws of dynamic friction

� Following are the laws of dynamic friction :� The friction force always act in a direction, opposite to that in which the body

is moving.

� The ratio of limiting friction (F) and normal reaction (N) is constant & it isknown as co-efficient of friction (µ).

F/N = µ=ConstantF/N = µ=Constant

� For moderate speeds, the friction force remains constant. But, it decreasesslightly with the increase of speed.

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Ladder FrictionLadder FrictionLadder FrictionLadder Friction

� The ladder is a device used to climb on thewall. The ladder is made of wood or iron.

� There are two long pieces in the ladder towhich many small cross pieces called rungsare connected at equal distances.are connected at equal distances.

� If the ladder is uniform throughout its lengththen the C.G. of the ladder will be at the mid-point of it.

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1. WL ( ) Weight of the ladder through C.G. of ladder [G]2. WM ( ) Weight of the man through [M], where man is standing.3. P ( ) Applied force P through [C] at a height h from the floor.4. RA ( ) Reaction by the floor on the ladder.5. FA ( ) Friction force on the ladder at point A. Ladder slips rightward on the floor and hence FA is leftward.6. RB ( ) Reaction by the wall on the ladder. 7. FB ( ) Friction force on the ladder at point B. Ladder slips downward on the wall and hence FB is upward.

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� A 5 metre long ladder, weighing 750 N is resting as shown in the fig. Check whetherladder will be stable or not. Suggest options to make it stable, if it is unstable. Thevertical wall is smooth and floor has coefficient of friction 0.25.

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Wedge FrictionWedge FrictionWedge FrictionWedge Friction

� Wedges are simple machines used to raise large stone blocksand other heavy loads.

� These loads can be raised by applying to the wedge a forceusually considerably smaller than the weight of the load.

� In addition, because of the friction between the surfaces in

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In addition, because of the friction between the surfaces incontact, a properly shaped wedge will remain in place afterbeing forced under the load.

� Wedges can thus be used advantageously to make smalladjustments in the position of heavy pieces of machinery.

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Wedges are used as shown to split tree trunks because thenormal forces exerted by the wedges on the wood are muchlarger than the forces required to insert the wedges.

WEDGE FRICTIONWEDGE FRICTIONWEDGE FRICTIONWEDGE FRICTION

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� Problems of wedges are the problems of equilibrium of forces.

� When wedge is inserted frictional forces are generated on the rubbingsurfaces of the wedge.

� Problems of equilibrium of forces acting on the wedge are solved by anyoneof the following three method.

1. Lami'sTheorem

2.By taking (i) ∑H = 0 (ii) ∑V = 0 i.e., by resolution of forces.

3.Graphical Method

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Thank YouThank You

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