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Mechanical Interview Questions 5 contains questions about Truss Bridges, I-section  heavy loading,  Center of Mass,  Center of Gravity,  Center of Mass,  Centroid, Projectile motion, Rocket motion, cooling used in High Voltage Transformer, Cylinder Liners,  Yield Stress, Yield Strength.

41. Why do you have Truss Bridges?

43. What is difference between Center of Mass and Center of Gravity?

44. What is the difference between Center of Mass and Centroid?

45. What are the differences between Weight and Mass?

46. What is the difference between Projectile motion and a Rocket motion?

47. What type of cooling used in High Voltage Transformer?

48. What is honing of Cylinder Liners?

49. What is the difference between Speed and Velocity?

50. Difference between Yield Stress and Yield Strength?

41. Why do you have Truss Bridges?

Truss bridges carry load in tension and compression rather than bending (Self weight + The weight of vehicles crossing it + Wind loads).

A truss has the ability to dissipate a load through the truss work. The beams are usually arranged in a repeated triangular pattern, since a triangle cannot be distorted by stress.

Cross sectional shape I, giving many benefits.

It is very good for giving stiffness (less deformation on loading) and to withstand higher bending moments (as a result of heavy loading) on comparison with other cross- sectional shapes of same area.

Also, it is very easy to manufacture. It will have more moment of inertia.

43. What is difference between Center of Mass and Center of Gravity?

Both terms are same when gravity is uniform. When gravity is non-uniform following are the terms:

The center of mass is a point that acts as if all the mass was centered there (the mass on one side of the point is equal to the mass on the opposite side). If supported at the center of mass, an object will be balanced under the influence of gravity.

The center of gravity is the point at which where the sum (vector) of the gravitational forces act on an object which will be balanced on that point.

44. What is the difference between Center of Mass and Centroid?

If the material composing a body is uniform or homogeneous, the density or specific weight will be constant throughout the body, and then the centroid is the same as the center of gravity or center of mass.

Centroid: Centroid is the point, where the whole area of plane is going to be act. It is valid only for 2D problems like plane figures, square plate etc.

The center of mass is a point that acts as if all the mass was centered there (the mass on one side of the point is equal to the mass on the OPPOSITE side).

If supported at the center of mass, an object will be balanced under the influence of gravity.

45. What are the differences between Weight and Mass?

Mass is a measure of how much matter an object has. Mass is specified in Gram or Kilograms.

Weight is a measure of how strongly gravity pulls on that matter. Force is measured in Newton’s.

F=m.g

Thus if you were to travel to the moon your weight would change because the pull of gravity is weaker there than on Earth but, your mass would stay the same because you are still made up of the same amount of matter.

46. What is the difference between Projectile motion and a Rocket motion?

A projectile has no motor/rocket on it, so all of its momentum is given to it as it is launched. An example of a projectile would be pen that you throw across a room.

A rocket or missile does have a motor/rocket on it so it can accelerate itself while moving and so resist other forces such as gravity.

In mechanics point of view projectile don’t have any particular shape it is a point mass.

Whereas rocket has a particular shape and hence it has center of gravity situated at particular point on its body. Therefore rocket motion comes under kinetics and projectile comes under kinematics.

47. What type of cooling used in High Voltage Transformer?

The big transformers you find on power poles usually use oil as a dielectric insulator, in smaller HV transformers, like the ones in TV’s are usually filled with resin.

48. What is honing of Cylinder Liners?

The honing equipment used has been manufactured by "Chris Marine". The head of the honing device consists of four synchronized stones.

For the initial honing diamond stones are used to break up the hardened surface in the scuffed areas. For the main honing very coarse and hard stones are used to produce a very desirable rough surface all over the liner.

The advantage, especially for the 2-stroke engines, is possibility to save the liner after a seizure, scuffing or blow-by or even to eliminate the ovality of the liner.

Another advantage is that it is possible to machine a rough liner wall to obtain a well oiled surface.

49.What is the difference between Speed and Velocity?

Speed is scalar quantity and Velocity is a vector

– velocity has both speed and direction.

Speed is expressed as distance moved (d) per unit of time(t).

Speed is measured in the same physical units of measurement as velocity, but does not contain an element of direction. Speed is thus the magnitude component of velocity.

50. Difference between Yield Stress and Yield Strength?

Stress is a measure of the load applied to a sample relative to a cross sectional area of the sample. Strength is a quantification of the samples ability to carry a load.

The terms "yield strength" and "yield stress" of a material are usually used interchangeably (correct or not). It is the stress which will just cause the material to plastically deform.

If a material yields at 30,000 psi, the yield stress is 30,000 psi. If the part in question has a cross sectional area of 2 square inches, the strength at yield would be 60,000 pounds, but usually we just say the yield strength is 30,000 psi.