Category: Machine Kinematics Objective Questions

250+ TOP MCQs on Law of Gearing-1 and Answers

Machine Kinematics Multiple Choice Questions on “Law of Gearing – 1”.

1. The two parallel and coplaner shafts are connected by gears having teeth parallel to the axis of the shaft. This arrangement is known as
a) spur gearing
b) helical gearing
c) bevel gearing
d) spiral gearing
Answer: a
Clarification: The two parallel and co-planar shafts connected by the gears. These gears are called spur gears and the arrangement is known as spur gearing.

2. The arrangement is called bevel gearing, when two __________ are connected by gears.
a) tension in the tight side of the belt
b) tension in the slack side of the belt
c) sum of the tensions on the tight side and slack side of the belt
d) average tension of the tight side and slack side of the belt
Answer: a
Clarification: The two non-parallel or intersecting, but coplanar shafts connected by gears. These gears are called bevel gears and the arrangement is known as bevel gearing.

3. When two non-intersecting and non-coplaner shafts are connected by gears,the arrangement is known as helical gearing.
a) True
b) False
Answer: b
Clarification: The two parallel and co-planar shafts connected by the gears. These gears are called spur gears and the arrangement is known as spur gearing. These gears have teeth parallel to the axis of the wheel. Another name given to the spur gearing is helical gearing.

4. The gears are termed as medium velocity gears, if their peripheral velocity is
a) 1-3 m/s
b) 3-15 m/s
c) 15-30 m/s
d) 30-50 m/s
Answer: b
Clarification: The gears having velocity less than 3 m/s are termed as low velocity gears and gears having velocity between 3 and 15 m/s are known as medium velocity gears.

5. An imaginary circle which by pure rolling action, gives the same motion as the actual gear, is called
a) addendum circle
b) dedendum circle
c) pitch circle
d) clearance circle
Answer: c
Clarification: Addendum circle is the circle drawn through the top of the teeth and is concentric with the pitch circle.
Dedendum circle is the circle drawn through the bottom of the teeth. It is also called root circle.
Pitch circle is an imaginary circle which by pure rolling action, would give the same motion as the actual gear.

6. The size of a gear is usually specified by
a) pressure angle
b) circular pitch
c) diametral pitch
d) pitch circle diameter
Answer: d
Clarification: Pitch circle diameter is the diameter of the pitch circle. The size of the gear is usually specified by the pitch circle diameter. It is also known as pitch diameter.

7. The radial distance of a tooth from the pitch circle to the bottom of the tooth is called
a) dedendum
b) addendum
c) clearance
d) working depth
Answer: a
Clarification: Dedendum is the radial distance of a tooth from the pitch circle to the bottom of the tooth.
Addendum is the radial distance of a tooth from the pitch circle to the top of the tooth.

8. The addendum is the radial distance of tooth from the pitch circle to the top of the tooth.
a) True
b) False
Answer: a
Clarification: Dedendum is the radial distance of a tooth from the pitch circle to the bottom of the tooth.
Addendum is the radial distance of a tooth from the pitch circle to the top of the tooth.

9. The working depth of a gear is radical distance from the
a) pitch circle to the bottom of a tooth
b) pitch circle to the top of a tooth
c) top of a tooth to the bottom of a tooth
d) addendum circle to the clearance circle
Answer: d
Clarification: Working depth is the radial distance from the addendum circle to the clearance circle. It is equal to the sum of the addendum of the two meshing gears.

10. The radial distance from the top of a tooth to the bottom of a tooth in a meshing gear, is called
a) dedendum
b) addendum
c) clearance
d) working depth
Answer: c
Clarification: Clearance is the radial distance from the top of the tooth to the bottom of the tooth, in a meshing gear. A circle passing through the top of the meshing gear is known as clearance circle.

250+ TOP MCQs on Standard Spur Gears – 2 and Answers

Machine Kinematics Assessment Questions and Answers on “Standard Spur Gears – 2”.

1. A spur gear of 40 teeth is machined in a gear hobbing machine using a double start hob cutter. The speed ratio between the hob and the blank is
a) 1:20
b) 1:40
c) 40:1
d) 20:1
Answer: d
Clarification: None.

2. A helical gear has the active face width equal to b pitch p and helix angle α. What should be the minimum value of b in order that contact is maintained across the entire active face of the gear?
a) p cosα
b) p secα
c) p tanα
d) p cotα
Answer: d
Clarification: b > p/tanα
b > p cotα.

3. Match the type of gears with their most appropriate description.
Type of gear Description

P. Helical 1. Axes non parallel and intersecting
Q. Spiral 2. Axes parallel and teeth are inclined to the axis
R. Hypoid 3. Axes parallel and teeth are parallel to the axis
S. Rack and pinion 4. Axes are perpendicular and intersecting,and teeth are inclined to the axis
5. Axes are perpendicular and used for large speed reduction
6. Axes parallel and one of the gears has infinite radius
a) P-2, Q- 4, R- 1, S- 6
b) P-1, Q- 4, R- 5, S- 6
c) P-2, Q- 6, R- 4, S- 2
d) P-6, Q- 3, R- 1, S- 5
Answer: a
Clarification: Helical – Axes parallel and teeth are inclined to the axis
Spiral – Axes are perpendicular and intersecting,and teeth are inclined to the axis
Hypoid – Axes non parallel and intersecting
Rack and pinion – Axes parallel and one of the gears has infinite radius.

4. Tooth interference in an external in volute spur gear pair can be reduced by
a) Decreasing center distance between gear pair
b) Decreasing module
c) Decreasing pressure angle
d) Increasing number of gear teeth
Answer: d
Clarification: There are several ways to avoid interfering:
i. Increase number of gear teeth
ii. Modified involutes
iii. Modified addendum
iv. Increased centre distance .

5. Interference in a pair of gears is avoided, if the addendum circles of both the gears intersect common tangent to the base circles within the points of tangency.
a) True
b) False
Answer: a
Clarification: None.

6. Twenty degree full depth involute profiled 19-tooth pinion and 37-tooth gear are in mesh. If the module is 5 mm, the centre distance between the gear pair will be
a) 140 mm
b) 150 mm
c) 280 mm
d) 300 mm
Answer: a
Clarification: Centre distance = D1 + D2/2 = mT1 + mT2/2 = 5(19 + 37)/2
= 140 mm.

7. If the drive efficiency is 80%, then torque required on the input shaft to create 1000 N output thrust is
a) 20 Nm
b) 25 Nm
c) 32 Nm
d) 50 Nm
Answer: b
Clarification: Module m = 2,
D/T = 2
∴ D = 80 × 2 = 160 mm
2F = 1000, or F = 500 N
Let T1 be the torque applied by motor.
TT2 be the torque applied by gear.
∴ Power transmission = 80%

Now, T1ω1= 2T2 x ω2/0.8
T1 = 25 N-m.

8. If the pressure angle of the rack is 20°, then force acting along the line of action between the rack and the gear teeth is
a) 250 N
b) 342 N
c) 532 N
d) 600 N
Answer: c
Clarification: Pcos φ = F
∴ Force acting along the line of action,
P = F/cos φ
= 500/cos20°
= 532 N.

9. Two mating spur gears have 40 and 120 teeth respectively. The pinion rotates at 1200 rpm and transmits a torque of 20 Nm. The torque transmitted by the gear is
a) 6.6 Nm
b) 20 Nm
c) 40 Nm
d) 60Nm
Answer: d
Clarification: We know, N1/N2 = T1/T2
N1 = speed of pinion
N2 = speed of gear wheel
T1 = number of teeth of gear
T2 = number of teeth of pinion

1200/N2 = 120/40
N2 = 400 r.p.m.
Since power transmitted by both gear will be equal
T1ω1= T2 x ω2

torque transmitted by gear, = 60 N-m.

10. The velocity ratio between pinion and gear in a gear drive is 2.3, the module of teeth is 2.0 mm and sum of number of teeth on pinion and gear is 99. What is the centre distance between pinion and the gear?
a) 49.5 mm
b) 99 mm
c) 148.5 mm
d) 198 mm
Answer: b
Clarification: Centre distance = D1 + D2/2 = mT1 + mT2/2
= m/2(T1 + T2)
= 2/2 x 99 = 99 mm.

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250+ TOP MCQs on Compound Pendulum and Answers

Machine Kinematics Multiple Choice Questions on “Compound Pendulum”.

1. Which of the following shape of the body can be considered as compound pendulum?
a) Cylindrical
b) Cubical
c) Cuboidal
d) Any rigid body
Answer: d
Clarification: Any rigid body having mass when suspended vertically and oscillates with small amplitude under the force of gravity undergoes SHM as a compound pendulum.

2. Compound pendulum needs to be spherical in shape.
a) True
b) False
Answer: b
Clarification: Any rigid body when suspended vertically, and it oscillates with a small amplitude under the action of the force of gravity, the body is known as compound pendulum. The rigid body is asssumed to have a mass and hence mass moment of inertia.

3. If the mass of the object is doubled then what will be the effect of time period of the compound pendulum?
a) Doubled
b) Remains same
c) Halved
d) Decreases by √2 times
Answer: d
Clarification: The time period of a compound pendulum is given by:
(2π)(sqrt{(frac{I}{mgl})}), now the dependency of the time period on the mass is inversely proportional to the root of mass, hence it decreases by √2 times.

4. If the mass moment inertia of the object is increased to 4 times then what will be the effect of time period of the compound pendulum?
a) Doubled
b) Remains same
c) Halved
d) Decreases by √2 times
Answer: a
Clarification: The time period of a compound pendulum is given by:
(2π)(sqrt{(frac{I}{mgl})}), now the dependency of the time period on the mass moment of inertia is directly proportional to the root of Moment of Inertia, hence it increases by 2 times.

5. Calculate the time period of an object having mass moment of inertia = 100 Kg-m2, mass of 10 Kg and the centre of gravity lies at a point 20 cm below the point of suspension.
a) 14.1
b) 15.2
c) 13.3
d) 12.9
Answer: a
Clarification: From the given data it is clear that the given rigid body constitutes a compound pendulum, now the time period of a compound pendulum is given by:
(2π)(sqrt{(frac{I}{mgl})})
substituting the values we get
T = 14.1 sec.

6. Calculate the frequency of vibration in Hz of an object having mass moment of inertia = 100 Kg-m2, mass of 10 Kg and the centre of gravity lies at a point 20 cm below the point of suspension.
a) 0.14
b) 0.15
c) 0.13
d) 0.07
Answer: d
Clarification: From the given data it is clear that the given rigid body constitutes a compound pendulum, now the time period of a compound pendulum is given by:
(2π)(sqrt{(frac{I}{mgl})})
substituting the values we get
T = 14.1 sec.
f = 1/T = 0.07 Hz.

7. Calculate the moment of inertia of an object having time period = 14 sec, mass of 10 Kg and the centre of gravity lies at a point 20 cm below the point of suspension.
a) 94.1
b) 95.2
c) 93.3
d) 97.4
Answer: d
Clarification: From the given data it is clear that the given rigid body constitutes a compound pendulum, now the time period of a compound pendulum is given by:
(2π)(sqrt{(frac{I}{mgl})})
substituting the values we get
I = 97.4 Kg-m2.

8. All simple pendulums are compound pendulums.
a) True
b) False
Answer: a
Clarification: Simple pendulum also is a compound pendulum where the mass is a point mass, hence the distance from centre of gravity is the length of the string.

9. If the mass moment inertia of the object is increased to 4 times then what will be the effect of time period of the simple pendulum?
a) Doubled
b) Remains same
c) Halved
d) Decreases by √2 times
Answer: b
Clarification: The time period of a simple pendulum is given by:
(2π)√(I/g), now the dependency of the time period on the mass moment of inertia is non existent, hence it remains unchanged.

10. Calculate the moment of inertia of an object having time period = 14 sec, mass moment of inertia= 100 Kg-m2 and the centre of gravity lies at a point 20 cm below the point of suspension.
a) 9.1
b) 9.2
c) 9.3
d) 10.2
Answer: d
Clarification: From the given data it is clear that the given rigid body constitutes a compound pendulum, now the time period of a compound pendulum is given by:
(2π)(sqrt{(frac{I}{mgl})})
substituting the values we get
m = 10.2 Kg.

250+ TOP MCQs on Inversion of Mechanism and Answers

Machine Kinematics Multiple Choice Questions on “Inversion of Mechanism”.

1. The number of degrees of freedom of a planer linkage with 8 links and 9 simple revolute joints is
a) 1
b) 2
c) 3
d) 4

Answer: c
Clarification: No. of links, I = 8
No. of revolute joints, J = 9
No. of higher pair, h = 0

Number of degree of freedom, n = 3(I -1) – 2j – h
= 3(8 – 1) – 2×9 – 0 = 3.

2. A palnar mechanism has 8 links and 10 rotary joints. The number of degrees of freedom of the mechanism, using Grubler’s criterion, is
a) 0
b) 1
c) 2
d) 3

Answer: b
Clarification: Whatever may be the number of links and joints Grubler’s criterion applies to mechanism with only single degree freedom. Subject to the condition 3I – 2J – 4 = 0 and it satisfy this condition.
Degree of freedom is given by = 3(I – 1) – 2j = 3(8 -1) – (2×10) = 1.

3. Match the approaches given below to perform stated kinematics/dynamics analysis of machine.
Analysis Approach
P. Continuous relative rotation 1. D’Alembert’s principle
Q. Velocity and acceleration 2. Grubler’s criterion
R. Mobility 3. Grashof’s law
S. Dynamic-static analysis 4. Kennedy’s theorem
a) P-1,Q-2,R-3,S-4
b) P-3,Q-4,R-2,S-1
c) P-2,Q-3,R-4,S-1
d) P-4,Q-2,R-1,S-3

Answer: b
Clarification:
1. D’Alembert’s principle – Dynamic-static analysis
2. Grubler’s criterion – Mobility(for a plane mechanism)
3. Grashof’s law – Continuous relative rotation
4. Kennedy’s theorem – Velocity and acceleration.

4. Which of the following statements is incorrect
a) Grashof’s rule states that for a planar crank-rocker four bar mechanism, the sum of the shortest and longest link lengths cannot be less than the sum of the remaining two link lengths.
b) Inversions of a mechanism are created by fixing different links one at a time.
c) Geneva mechanism is an intermittent motion device.
d) Gruebler’s criterion assumes mobality of a planar mechanism to be one.

Answer: a
Clarification: According to Grashof’s rule for complete relative rotation r/w links L + S < p + q.

5. In a four bar linkage, s denotes the shortest link length, L is the longest link length, P and Q are the lengths of other two links. At least one of the three moving links will rotate by 360° if
a) S + L < P + Q
b) S + L > P + Q
c) S + P < L + Q
d) S + P > L + Q

Answer: a
Clarification: According to Grashof’s law for four bar mechanism, the sum of shortest and longest link lengths should not be greater than the sum of remaining two link length.
i.e. S + L < P + Q.

6. The number of inversions for a slider crank mechanism is
a) 6
b) 5
c) 4
d) 3

Answer: c
Clarification: No. of links of a slider crank mechanism = 4
So there are four inversion of slider crank mechanism.

7. The mechanism used in a shaping machine is
a) A closed 4bar chain having 4 revolute pairs
b) A closed 6bar chain having 6 revolute pairs
c) A closed 4bar chain having 2 revolute and 2 sliding pairs
d) An inversion of single slider crank chain

Answer: d
Clarification: Quick return mechanism.

8. Match the following
Types of mechanism Motion achieved
P. Scott Russel mechanism 1. Intermittent motion
Q. Geneva mechanism 2. Quick return motion
R. Off set slider crank mechanism 3. Simple harmonic motion
S. scotch Yoke mechanism 4. Straight line motion
a) P-2,Q-3,R-1,S-4
b) P-3,Q-2,R-4,S-1
c) P-4,Q-1,R-2,S-3
d) P-4,Q-3,R-1,S-2

Answer: c
Clarification:
1. Intermittent motion – Geneva mechanism
2. Quick return motion – Off set slider crank mechanism
3. Simple harmonic motion – scotch Yoke mechanism
4. Straight line motion – Scott Russel mechanism.

9. In a kinematic chain, a quaternary joint is equivalent to
a) one binary joint
b) two binary joint
c) three binary joint
d) four binary joint

Answer: c
Clarification: When ‘l’ number of links are joined at the same connection, the joint is equivalent to (l – 1) binary joints.

10. When supported on three points, out of 12 degrees of freedom the number of degrees of freedom arrested in a body is
a) 3
b) 4
c) 5
d) 6

Answer: d
Clarification: When supported on three points, following six degrees of freedom are arrested.

250+ TOP MCQs on Forces Acting in a Mechanism and Answers

Machine Kinematics Multiple Choice Questions on “Forces Acting in a Mechanism”.

1. Coriolis component of acceleration is a component of translatory acceleration.
a) True
b) False
Answer: a
Clarification: Its unit is m/s2. Therefore translatory acceleration (at = 2ωV).

2. If the relative motion between two links of a mechanism is pure sliding, then the relative instantaneous center for these two links does not exist.
a) True
b) False
Answer: b
Clarification: It does exists at infinity distance. Kennedy theorem says number of instantaneous center N = n(n – 1)/2

3. A slider sliding at 10 cm/s on a link which is rotating at 60 r.p.m. is subjected to Coriolis acceleration of magnitude
a) 40п2 cm/s2
b) 0.4пcm/s2
c) 40пcm/s2
d) 4пcm/s2
Answer: c
Clarification: Coriolis acceleration = 2ωV = 2 x 2пN/60 x V = 2 x 2п x 60/60 x 10 = 40пcm/s2

4. A body in motion will be subjected to coriolis acceleration when that body is
a) in plane rotation with variable velocity
b) in plane translation with variable velocity
c) in plane motion which is a resultant of plane translation and rotation
d) restrained to rotate while sliding over another body
Answer: d
Clarification: When a point on one link is sliding along another rotating link, such as in quick return motion mechanism, then the coriolis component of the acceleration must be calculated.

5. Mechanism used to produce a diagram to an enlarged or reduced sacle
a) hart’s mechanism
b) pantograph
c) grasshopper mechanism
d) peaucellier’s mechanism
Answer: b
Clarification: Pantograph mechanism is used to produce a diagram to an enlarged or reduced sacle.
Hart’s mechanism is used to produce exact straight line motion mechanism.

6. A straight line mechanism made up of turning pairs
a) hart’s mechanism
b) pantograph
c) grasshopper mechanism
d) none of the mentioned
Answer: a
Clarification: Exact straight line motion mechanisms made up of turning pairs are peaucellier’s mechanism and hart’s mechanism.

7. Approximate straight line motion consisting of one sliding pair
a) hart’s mechanism
b) pantograph
c) grasshopper mechanism
d) peaucellier’s mechanism
Answer: c
Clarification: Hart’s mechanism is used to produce exact straight line motion mechanism.
Grasshopper mechanism gives approximate straight line motion consisting of one sliding pair.

8. Exact straight line motion mechanism
a) hart’s mechanism
b) pantograph
c) grasshopper mechanism
d) none of the mentioned
Answer: a
Clarification: Exact straight line motion mechanisms made up of turning pairs are peaucellier’s mechanism and hart’s mechanism.

9. The sense of Coriolis component 2ωV is the same as that of the relative velocity vector V rotated.
a) 450 in the direction of rotation of the link containing the path
b) 450 in the direction opposite to the rotation of the link containing the path
c) 900 in the direction of rotation of the link containing the path
d) 1800 in the direction opposite to the rotation of the link containing the path
Answer: c
Clarification: The sense of Coriolis component 2ωV is the same as that of the relative velocity vector V rotated 900 in the direction of rotation of the link containing the path.
The direction of coriolis component is along a line rotated 900 from the sliding velocity in a direction same as that of the angular velocity of the slotted lever.

10. What is the direction of the coriolis component of acceleration in a slotted lever-crank mechanism?
a) Along the sliding velocity vector
b) Along the direction of the crank
c) Along a line rotated 900 from the sliding velocity vector in a direction opposite to the angular velocity of the slotted lever
d) Along a line rotated 900 from the sliding velocity in a direction same as that of the angular velocity of the slotted lever.
Answer: d
Clarification: The sense of Coriolis component 2ωV is the same as that of the relative velocity vector V rotated 900 in the direction of rotation of the link containing the path.
The direction of coriolis component is along a line rotated 900 from the sliding velocity in a direction same as that of the angular velocity of the slotted lever.

250+ TOP MCQs on Limiting Angle of Friction and Answers

Machine Kinematics Multiple Choice Questions on “Limiting Angle of Friction”.

1. When the rubbing surfaces are separated from each other by a very thin film of lubricant it is known as
a) kinetic friction
b) dry friction
c) boundary lubrication
d) none of the mentioned

Answer: c
Clarification: When the rubbing surfaces are separated from each other by a very thin film of lubricant it is known as boundary lubrication.
When the two solid surfaces sliding over each other are free from contaminating fluid or film, the resistance encountered is called dry friction.

2. Sliver sulphate, tungsten disulphide, graphite, molybdenum disulphide, lead oxide are examples of
a) boundary lubrication
b) viscous lubrication
c) solid lubrication
d) none of the mentioned

Answer: c
Clarification: Sliver sulphate, tungsten disulphide, graphite, molybdenum disulphide, lead oxide are examples of solid lubrication.
Step bearing is used to resist the end thrust of the shafts.

3. __________ is used to resist the end thrust of the shafts.
a) friction circle
b) step bearings
c) injector
d) none of the mentioned

Answer: b
Clarification: Step bearing is used to resist the end thrust of the shafts.

4. When the two solid surfaces sliding over each other are free from contaminating fluid or film, the resistance encountered is called
a) kinetic friction
b) dry friction
c) boundary lubrication
d) none of the mentioned

Answer: b
Clarification: When the two solid surfaces sliding over each other are free from contaminating fluid or film, the resistance encountered is called dry friction.
When the rubbing surfaces are separated from each other by a very thin film of lubricant it is known as boundary lubrication.

5. For design of ordinary machinery, value of journal friction can be taken as
a) 0.008 to 0.02
b) 0.07 to 0.15
c) 0.05 to 0.80
d) none of the mentioned

Answer: a
Clarification: For very low velocities of rotation, high loads, and with good lubrication can be taken as 0.07 to 0.15.
For design of ordinary machinery, value of journal friction can be taken as 0.008 to 0.02.

6. For very low velocities of rotation, high loads, and with good lubrication can be taken as
a) 0.008 to 0.02
b) 0.07 to 0.15
c) 0.05 to 0.80
d) none of the mentioned

Answer: b
Clarification: For very low velocities of rotation, high loads, and with good lubrication can be taken as 0.07 to 0.15.
For design of ordinary machinery, value of journal friction can be taken as 0.008 to 0.02.

7. When the lubrication is arranged so that rubbing surfaces are separated by a fluid film, and the load on the surfaces is carried entirely by the hydrostatic or hydrodynamic pressure in the film, it is known as
a) boundary lubrication
b) viscous lubrication
c) solid lubrication
d) none of the mentioned

Answer: b
Clarification: When the lubrication is arranged so that rubbing surfaces are separated by a fluid film, and the load on the surfaces is carried entirely by the hydrostatic or hydrodynamic pressure in the film, it is known as viscous lubrication.
When the rubbing surfaces are separated from each other by a very thin film of lubricant it is known as boundary lubrication.

8. When the load on the rubbing surfaces is carried partly by a fluid viscous film and partly by areas of boundary lubrication, it is known as
a) incomplete lubrication
b) viscous lubrication
c) solid lubrication
d) none of the mentioned

Answer: a
Clarification: When the lubrication is arranged so that rubbing surfaces are separated by a fluid film, and the load on the surfaces is carried entirely by the hydrostatic or hydrodynamic pressure in the film, it is known as viscous lubrication.
When the rubbing surfaces are separated from each other by a very thin film of lubricant it is known as boundary lubrication.
When the load on the rubbing surfaces is carried partly by a fluid viscous film and partly by areas of boundary lubrication, it is known as incomplete lubrication.

9. When effect of friction is negligible, the force is transmitted by the link from driver to the driven link through the centre line of the pins connecting the link. With friction, the line of action shifts and is tangent to _____________
a) kinetic friction
b) dry friction
c) friction circle
d) none of the mentioned

Answer: c
Clarification: When effect of friction is negligible, the force is transmitted by the link from driver to the driven link through the centre line of the pins connecting the link. With friction, the line of action shifts and is tangent to friction circle.
When the load on the rubbing surfaces is carried partly by a fluid viscous film and partly by areas of boundary lubrication, it is known as incomplete lubrication.

10. If one surface slides over the other, being pressed together by a normal force N, a frictional force F resisting the motion must be overcome. The ratio F/N is called ______________
a) kinetic friction
b) dry friction
c) friction circle
d) none of the mentioned

Answer: a
Clarification: If one surface slides over the other, being pressed together by a normal force N, a frictional force F resisting the motion must be overcome. The ratio F/N is called kinetic friction.
When the two solid surfaces sliding over each other are free from contaminating fluid or flim, the resistance encountered is called dry friction.