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ISRO Mechanical Engineering Mechanics Questions, Answers  and Explanation Kinematics of Motion 2006. 37. A person walks up a stalled escalator in 90 seconds. When standing on the same escalator, now moving, he is carried up in 60 seconds. How much time would it take him to walk up the moving escalator.      a) 30 sec      b) 36 sec      c) 45 sec      d) 54 sec Answer b) 36 sec Explanation Let the distance the person has to cover be d. Let the velocity of the person be v m . Let the velocity of the moving escalator be v e When the escalator is stalled the person walks with velocity v m . Use distance = velocity x time d = v m x 90  ---- 1 When standing on the moving escalator the person moves with velocity v e . d = v e x 60 ----- 2 Dividing 1 by 2 we get 1 = v m x 90 /(v e x 60) v m = 2v e /3 ---- 3 Let the time taken for him to walk up the moving escalator be t seconds. When wal...

Strength of Materials Simple Stresses and Strains 2017.2.9.  Which of the following is true for ductile materials? a) Engineering stress – strain curve cannot have negative slope b) Most applicable failure theory is maximum principal stress theory c) Ultimate strain is the strain at ultimate stress d) Strain hardening is represented by a negative slope in engineering stress strain curve Answer c)  Ultimate strain is the strain at ultimate stress Explanation  Principal stress theory is applicable for brittle materials. Elastic Constants 2017.2. 2.  For isotropic materials, the modulus of Elasticity in tension and shear (E and G) are related to the Poisson's ratio (&nu) as follows a) E = G/(2(1+ν)) b) G = E/(2(1+ν)) c) G = E/(2(1-ν)) d) E = G/(2(1-ν)) Answer b) G = E/(2(1+ν)) Explanation Derivation of relation between Modulus of elasticity in tension and shear Torsion 2017.2.76.  A 3m l...

ISRO Mechanical Engineering Manufacturing Technology Questions, Answers and Explanation Welding 2018.10.  In a linear arc welding process, the heat input per unit length is inversely proportional to  a) welding current b) welding speed c) welding voltage d) duty cycle of power source Answer b) welding speed Explanation Welding power gives the heat input per unit length. Welding current and welding voltage is directly proportional to welding power and hence directly proportional to the heat input per unit length.  Duty cycle is the ratio of arcing time to the weld cycle time multiplied by 100. Welding cycle time is either 5 minutes as per European standards or 10 minutes as per American standard and accordingly power sources are designed. If arcing time is continuously 5 minutes then as per European standard it is 100% duty cycle and 50% as per American standard. At 100% duty cycle minimum current is to be drawn i.e. with the reduction of duty...