This set of Engineering Mechanics Questions and Answers for Entrance exams focuses on “Frictional Forces on Collar Bearing, Pivot Bearings and Disks”.

1. The pivots are generally used in __________

a) Belts

b) Columns

c) Beams

d) Machines

View Answer

Explanation: The pivots are being used in the machines. They are used as to support the loadings. This means that the more the pivots the more is the stable the structure. Thus the use of pivots.

2. Determine the moment about the point Q by the force shown as 400N.

a) -98.6kN

b) 98.6kN

c) -98.6iN

d) -98.6jN

View Answer

Explanation: As we know that the moment is the cross product of the force and the distance between the point of contact of the force and the point about which moment needs to be calculated. Thus forming the distance vector and then crossing it with the force will give us the answer. Remember force also needs to be in the vector form for doing the cross product.

3. The collar bearings are generally used in __________

a) Belts

b) Columns

c) Beams

d) Machines

View Answer

Explanation: The collar bearings are being used in the machines. They are used as to support the loadings. This means that the more the collar bearings the more is the stable the structure. Thus the use of collar bearings.

4. The pivots and collar bearings are used as to support the axial load on the rotating shaft.

a) False

b) True

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Explanation: The collar bearings and pivots are being used in the machines. They are used as to support the loadings. This means that the more the collar bearings or pivots more is the stable the structure. Thus used as a supporting system to the shafts.

5. A phenomena is there in these collar bearings and pivots, which is very helpful in the supporting of the machines, and that is _____________

a) Column Friction

b) Coulomb Friction

c) Dry column friction

d) Surface friction

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Explanation: The dry friction is acted upon the surfaces. And they are tangential to each other. As we know the friction is the phenomena that defines that there is a resistance which is present there between the two surfaces. This Coulomb friction is also known as dry friction.

6. At what conditions does the Coulomb friction occur between the surfaces in contact of collar bearings or pivots?

a) When there is no lubricating fluid

b) When there is no friction fluid

c) When there is no adhesive fluid

d) When there is no cohesive fluid

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Explanation: When there is no lubricating fluid present between the surfaces in contact, the dry friction occurs. Thus as the name suggests dry friction. When there is no liquid present over there the surfaces are going to be said dry only. Thus the term dry friction.

7. We first make equilibrium equations of the sections involving the collar bearings or pivots by considering all the three dimensional forces acting on the section chosen and then the free body diagram is made and solved.

a) The first part of the statement is false and other part is true

b) The first part of the statement is false and other part is false too

c) The first part of the statement is true and other part is false

d) The first part of the statement is true and other part is true too

View Answer

Explanation: We first make the free body diagram of collar bearings or pivots and then we make the equilibrium equations to satisfy the given conditions. This helps us to solve the question easily. As this reduces the part of imagination and increases accuracy too.

8. To know the rotations involved in the disks we use right handed coordinate system, in that system which of the following is followed (consider the mentioned axis to be positive)?

a) Thumb is z-axis, fingers curled from x-axis to y-axis

b) Thumb is x-axis, fingers curled from z-axis to y-axis

c) Thumb is y-axis, fingers curled from x-axis to z-axis

d) Thumb is z-axis, fingers curled from y-axis to x-axis

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Explanation: As right handed coordinate system means that you are curling your fingers from positive x-axis towards y-axis and the thumb which is projected is pointed to the positive z-axis. Thus visualizing the same and knowing the basic members of the axis will not create many problems.

9. In the calculations involving the collar bearings and the pivot bearings, we use cross product. In that mathematically, for two vectors A and B of any magnitude, the cross product of both, i.e. AxB = given by:

a) |A||B|sinØ

b) |A||B|

c) |A||B|cosØ

d) |A||B|sin(180°+Ø)

View Answer

Explanation: The cross product of two vectors gives a vector which is perpendicular to both of the vectors. And the mathematic equation for the same is given by: |A||B|sinØ. And the dot product of the same by any of the other two vectors will give the answer zero, as perpendicular. The cross product is the same for all the systems of its application.

10. In the calculations of the bearings, we often use vector math. So for those systems which of the following is true?

a) i x i =1

b) j x i = -j

c) k x j = -i

d) k x k = 1

View Answer

Explanation: As the mathematic equation for the cross product is having a cosine function in it, in which the angle used in the function is the angle between the vectors. Thus the cross product will be zero if the angle between them is 90. The vector math system remains the same for every system in the environment.

11. The moment is the cross product of which two vectors in the calculations of the pivots and disks?

a) Force and Radius vectors

b) Radius and Force vectors

c) Force and Radius scalars

d) Radius and Force scalars

View Answer

Explanation: The cross product needs to take in the proper sequence. If not taken then the answer is just the opposite of the true answer. That’s why, the answer is not the Force and Radius vectors, but the Radius and Force vectors. Because the moment has its direction, as many of the cross products have, and thus precaution needs to be taken.

12. Principle of transmissibility is somehow applied to the collar bearings. It states that:

a) It states that the force acting on the body is a sliding vector

b) It states that the force acting on the body is a rolling vector

c) It states that the force acting on the body is a wedging vector

d) It states that the force acting on the body is a unit vector

View Answer

Explanation: Principle of transmissibility states that the force acting on the body is a sliding vector. That is it can be applied at any point of the body. It will give the same effect as if applied at any point other that the specific point. Thus the answer.

13. Coplanar forces are not easily simplified in the simplification of the force and couple system in the calculations of forces in the pivots and collar bearings.

a) True

b) False

View Answer

Explanation: The coplanar forces can be easily simplified in the simplification of the force and couple system. This is not only for the coplanar system of the forces but for any of the dimension vector. Which means the answer is false.

14. Determine the moment about the point P.

a) 460Nm

b) 500Nm

c) 705Nm

d) 0Nm

View Answer

Explanation: As we know that the moment is the cross product of the force and the distance between the point of contact of the force and the point about which moment needs to be calculated. Thus forming the distance vector and then crossing it with the force will give us the answer. Remember force also needs to be in the vector form for doing the cross product.

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