frictionengineering mechanics (mee 202) 48 example a ladder of length l rests against a wall, the...
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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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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