Friction

Dr. Vishesh Ranjan Kar Assistant Professor Department of Mechanical Engineering NIT Jamshedpur

What is Friction?

• Force that acts oppose the relative motion of two surfaces

• High for dry and rough surfaces

• Low for smooth and wet surfaces

There are two types of friction: dry or Coulomb

friction and fluid friction. Fluid friction applies to

lubricated mechanisms. The present discussion is

limited to dry friction between non-lubricated

surfaces.

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Free Body Diagram Normal Force FN

Friction Force ff

Applied Force F

Gravity Force Fg

Fg = mg

FN = Fg

ff = F

Static Friction

frictionstaticoftcoefficienFf

s

sNs

=×=

µµ

FN

fs

F

Fg

The Force of Static Friction keeps a stationary object at rest

Kinetic Friction

frictionkineticoftcoefficienFf

k

kNk

=×=

µµ

FN

fk

F

Fg

Once the Force of Static Friction is overcome, the Force of Kinetic Friction is what slows down a moving object

Motion

The coefficients of friction are μ s = 0.40 and μ k = 0.30 between all

surfaces of contact. Determine the force P for which motion of the

30-kg block is impending if cable AB (a) is attached as shown, (b) is

removed.

Example

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A block weighing 36N is resting on a rough inclined plane having an inclination of 30º. A force of 12N is applied at an angle of 10º up the plane and the block is just on the point of moving down the plane. Determine the coefficient of friction.

Wedges

• Wedges - simple machines used to raise heavy loads.

• Force required to lift block is significantly less than block weight.

• Friction prevents wedge from sliding out.

• Want to find minimum force P to raise block.

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28

Determine the smallest horizontal force P required to pull out

wedge A. The crate has a weight of 300 N and the coefficient of

static friction of all containing surfaces is 0.3. Neglect the weight of

the wedge.

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LADDER FRICTION A ladder of length 4 m, weighing 200 N is placed against a vertical

wall as shown in Fig. The coefficient of friction between the wall and

the ladder is 0.2 and that between floor and the ladder is 0.3. The

ladder, in addition to its own weight, has to support a man weighing

600 N at a distance of 3 m from A. Calculate the minimum horizontal

force to be applied at A to prevent slipping.

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The ladder shown in Fig. is 6 m long and is supported by a horizontal floor and a vertical wall. The coefficient of friction between the floor and the ladder is 0.25 and between the wall and the ladder is 0.4. The weight of the ladder is 200 N and may be considered as a concentrated load at G. The ladder supports a vertical load of 900 N at C which is at a distance of 1 m from B. Determine the least value of α at which the ladder may be placed without slipping. Determine the reaction at that stage.

Example

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Engineering Mechanics (MEE 202) 48

Example A ladder of length L rests against a wall, the angle of inclination being

45°. If the coefficient of friction between the ladder and the ground

and that between ground and the wall is 0.5 each, what will be the

maximum distance on ladder to which a man whose weight is 1.5

times the weight of ladder may ascend before the ladder begins to

slip?

HW