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ISRO 2017 May Mechanical Questions, Answers and Explanation

ISRO 2017 May Mechanical Questions, Answers and Explanation

1. A solid rod of 12mm diameter was tested for tensile strength with the gauge length of 50mm. Final length = 80mm; Final diameter = 4mm; Yield load = 1130N. What is the nearest yield stress and % reduction in area respectively?
a) 10MPa and 10%
b) 90MPa and 90%
c) 10MPa and 90%
d) 90MPa and 10%

Answer
c) 10MPa and 90%

Explanation
Yield stress = Yield load/Area at yield
= 1130/(π (4x10-3)2/4)
=  4520x106/(16π)
% reduction in area = Change in area*100/Original area

2. When water at 0C is heated, its specific volume under standard atmosphere
a) First increases then decreases
b) First decreases then increases
c) Increases steadily
d) Decreases steadily

Answer
b) First decreases then increases

Explanation
The density of water is highest at 4C. That is the specific volume is minimum for water at 4C.

3. A spherical joint has ....... degree (s) of freedom.
a) One
b) Two
c) Three
d) Four

Answer
c) Three

Explanation
A limb connected to a spherical joint can move in all possible planes containing the joint.

4. Isometric projection of sphere is
a) Circle
b) Ellipse
c) Hyperbola
d) Parabola

Answer
a) Circle

5. ABC inventory control focuses on those
a) Items not readily available
b) Items which consume less money
c) Items which have more demand
d) Items which consume more money

Answer
d) Items which consume more money

6. How does the viscosity of fluids vary with the increase in temperature?
a) Increases for both liquids and gases
b) Increases for liquids, decreases for gases
c) Increases for gases, decreases for liquids
d) Decreases for both liquids and gases

Answer
c) Increases for gases, decreases for liquids

7. It is required to connect two parallel shafts, the distance between whose axes is small and variable. They can be coupled by
a) Universal joint
b) Knucle joint
c) Sleeve coupling
d) Oldham coupling

Answer
d) Oldham coupling

Explanation
Universal joint - axes are aligned to one another
Knuckle joint - axes in same plane, tensile load
Sleeve coupling - concentric locked by key
Oldham coupling - designed because of paddle placement problem

8. Under what condition does flow separation take place?
a) When dp/dx > 0
b) When dp/dx < 0
c) When dp/dx = 0
d) Not a function of dp/dx

Answer
a) When dp/dx > 0

Explanation
Flow separation takes place when there is an adverse pressure gradient, that is the fluid is not able to travel further in that direction where pressure increases.

9. For a specified inlet and outlet temperatures, for which kind of heat exchanger will the log mean temperature difference, del Tm will be the highest?
a) Double pipe parallel flow heat exchanger
b) Double pipe counter flow heat exchanger
c) Cross flow heat exchanger
d) Multi pass shell and tube heat exchanger

Answer
b) Double pipe counter flow heat exchanger

10. Fatigue strength for non-ferrous materials is usually defined at ...... stress cycles
a) 10^4
b) 10^7
c) 10^9
d) 10^12

Answer
b) 10^7

13. The solution of ODE ydx + xdy = 0 is
a) y = k/x
b) y = k/(y-x)
c) y = x/y
d) y = e^(kx)

Answer
a) y = k/x

Explanation
Separating the variables, dx/x = -dy/y
Integrating ln x = -ln y + ln k
ln x + ln y = ln k
xy = k, y = k/x

14. If phi is a scalar point function, the value of curl Grad phi is
a) 0
b) 1
c) 2
d) 3

Answer
a) 0

15. The point where the plane x+3y+2z-6=0 meets the line joining P(2,1,1) and Q(1,3,-2) is,
a) (2,1,1)
b) (1,1,1)
c) (1,3,-2)
d) (-1,3,2)

Answer
c) (1,3,-2)

Explanation
Equation of line joining (2,1,1) and (1,3,-2) is (x-1)/(2-1) = (y-3)/(1-3) = (z+2)/(1+2) = t
x-1 = (y-3)/(-2) = (z+2)/3 = t
Parametric equation of that line is (t+1,-2t+3,3t-2)
Substitute the parametric equation of line in the equation of plane to solve for t to get the point of intersection
t+1+3(-2t+3)+2(3t-2)-6 =0
t+1-6t+9+6t-4-6=0
t =0
The point where the plane meets the line is (0+1,-2*0+3,3*0-2) = (1,3,-2)

17. A centrifugal compressor is suitable for which of the following?
a) High pressure ratio, low mass flow
b) Low pressure ratio, low mass flow
c) High pressure ratio, high mass flow
d) Low pressure ratio, high mass flow

Answer
d) Low pressure ratio, high mass flow

19. A rod of length L and area of cross section A has a modulus of elasticity E and coefficient of thermal expansion a. One end of the rod is fixed and other end is free. If the temperature of the rod is increased by del T, then
a) Stress developed in the rod is EadelT and strain developed in the rod is adelT
b) Stress developed in the rod is zero and strain developed in the rod is -adelT
c) Stress developed in the rod is zero and strain developed in the rod is adelT
d) Stress developed in the rod is EadelT and strain developed in the rod is zero

Answer
c) Stress developed in the rod is zero and strain developed in the rod is adelT

Explanation
As one end is free there will be no stress developed in the rod

20. The entropy of a hot baked potato decreases as it cools
a) The above statement is correct
b) The above statement is incorrect as it violates the increase in entropy principle
c) Too less information to comment
d) The above statement is incorrect as it violates second law of thermodynamics

Answer
a) The above statement is correct

Explanation [Thermal Engineering]
Magnitude of increase of entropy of the surroundings is more than the magnitude of the decrease of entropy of the hot baked potato. As the overall entropy increases the process is not reversible and practical.

21. During January, at a location in Rohtang, winds at -6C can be observed. Several meters below the ground, the temperature remains at 27C. A scientist claims to have devised a power cycle exploiting this situation that has a thermal efficiency of 10%. Whether the claim is true?
a) True
b) False
c) Can't say
d) Insufficient data

Answer
a) True

Explanation [Thermal Engineering]
Thermal efficiency = 1 - T2/T1 = 1 - 267/300 = 33/300 = 11%. As his claim is less than the theoretical efficiency the claim is true

22. The side relief angle in a cutting tool signature of 10,20,7,6,8,15,2 is
a) 10
b) 7
c) 6
d) 8

Answer
c) 6

Explanation
10 - Back rake angle
20 - Side rake angle
7 - End relief angle
6 - Side relief angle
8 - End cutting edge angle
15 - Side cutting edge angle
2 - Nose radius

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