Quiz: Mechanical Engineering 5th July

Quiz: Mechanical Engineering
Exam: AE/JE
Topic: Miscellaneous

Each question carries 1 mark
Negative marking: 1/4 mark
Time: 15 Minutes

Q1. The damping force is constant in magnitude but opposite in direction to that of motion of vibrating bodies in case of…………………
(a) Viscous damping
(b) Coulomb damping
(c) Slip damping
(d) Structural damping

Q2. A shaft has two heavy rotors mounted on it. The transverse natural frequencies, considering each of the rotor separately, are 100 Hz and 200 Hz respectively. What would be the lowest critical speed?
(a) 5367 r.p.m
(b) 6000 r.p.m
(c) 9360 r.p.m
(d) 12000 r.p.m

Q3. The maximum force which acts on the connecting rod is
(a) Force due to gas pressure
(b) Force due to inertia of piston
(c) Force due to friction of connecting rod
(d) Force due to crank pin

Q4. Two parallel forces 100 kN and 125 kN act on a body and have resultant of 25 kN. Then the two forces are
(a) Like parallel forces
(b) Unlike parallel forces
(c) Concurrent forces
(d) None of the above

Q5. Strain rosettes are generally used for
(a) measurement of load
(b) measurement of shear strain
(c) measurement of longitudinal strain
(d) measurement of resilience

Q6. Stress-strain analysis is conducted to know which of the following properties of material?
(a) Physical properties
(b) Optical properties
(c) Mechanical properties
(d) Magnetic properties

Q7. A copper piece originally 305 mm long is pulled in tension with a stress of 276 MPa. If the deformation is entirely elastic and the modulus of elasticity is 110 GPa, the resultant elongation will be nearly
(a) 0.43 mm
(b) 0.54 mm
(c) 0.65 mm
(d) 0.77 mm

Q8. A cylindrical specimen of steel having an original diameter of 12.8 mm is tensile tested to fracture and found to have engineering fracture strength of 460 MPa. If its cross-sectional diameter at fracture is 10.7 mm, the true stress at fracture will be
(a) 660 MPa
(b) 645 MPa
(c) 630 MPa
(d) 615 MPa

Q9. The mechanism used to overcome a large resistance of a member with a small driving force is known as
(a) Indexing mechanism
(b) Snap action mechanism
(c) Whitworth mechanism
(d)Crank rocker mechanism

Q10. Toggle mechanism have following applications except
(a) Stone crushers
(b) Gear cutting
(c) Embossing presses
(d) Switches

Solution
S1. Ans.(b)
Sol. damping is defined as restraining of vibratory motion,
Types of damping-
(1) Viscous damping-
It is encountered by bodies moving at moderate speed through liquid. This type of damping leads to resisting force proportional to velocity. The damping force,
▭(F_d=cx ̇ ) c→Viscous damping coefficient
x ̇→ Velocity at any instant
(2) Coulomb damping –
This type of damping arises from sliding of dry surfaces. The friction force is nearly constant and depends upon the nature of sliding surface and normal pressure b/w them as expressed by the equation of kinetic friction.
F=μN
Where, μ= Coefficient of friction
N = Normal force
(3) Solid or structural damping –
This is due to the internal friction with in the material itself.
(4) Slip or interfacial damping –
Energy of vibration is dissipated by microscopic slip on the interfaces of machine parts in contact under fluctuating load.

S2. Ans.(a)
Sol. According to Drunkley’s method
When frequency of transvers vibration (f_n) is given
1/(f_n )^2 = 1/(f_n1 )^2 = 1/(f_n2 )^2
1/(f_n )^2 = 1/100^2 +1/200^2
1/(f_n )^2 =5/200^2
f_n=200/√5
=89.44 Hz
OR ▭(f_n=5367 rpm)

S3. Ans (a)
Sol. All of the given forces act on the connecting rod but the major force that acts on the connecting rod is the force due to gas pressure.

S4. Ans (b)
Sol. We know, according to parallelogram law,
R^2=√(P^2+Q^2+2PQ.cosθ) …………….(1)
Given,
R = 25 kN
P = 100 kN
Q = 125 kN
Then, from equation (1)
25=√(100^2+125^2+2×100×125.cosθ)
θ=180°

S5. Ans(c)
Sol. strain rosettes are generally used for measurement of longitudinal strain.
A strain gauge rosette is a term for an arrangement of two or more strain gauges that are positioned closely to measure strains along different directions of the component under evaluation. Single strain gauges can only measure strain effectively in one direction, so the use of multiple strain gauges enables more measurements to be taken, providing a more precise evaluation of strain on the surface being measured.

S6. Ans(c)
Sol. stress- strain analysis is conducted to know the Mechanical properties
of materials.

S7. Ans(d)
Sol. given that
stress (σ) = 276 MPa
length (L) = 305 mm
Modulus of Elasticity = 110 GPa
we know that, the resultant elongation is
δl=PL/AE
δl=σL/E
δl=(276×10^6×0.305)/(110×10^9 )
δl= 0.765 mm
δl= 0.77 mm

S8. Ans(a)
Sol. given that,
Initial diameter (d_i) = 12.8 mm
Final diameter (d_f) = 10.7 mm
Engineering stress (σ_e) = 460 MPa
True stress (σ_t) = ?
We know that,
σ_t/σ_e =A_i/A_f
σ_t=σ_e (A_i/A_f )
σ_t=σ_e (〖d_i〗^2/〖d_o〗^2 )
σ_t=460×(〖12.8〗^2/〖10.7〗^2 )
σ_t=658.27 MPa

S9. Ans (b)
Sol. It is a quick-break mechanism without intermediate state. Switching is obtained by the deformation of a spring or moving contact when the switch is operated.

S10. Ans (b)
Sol. Stone crushers, embossing presses and switches works on toggle mechanism. Mechanical advantage of toggle mechanism is infinite or very large.

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