This set of Engineering Mechanics Interview Questions and Answers for Experienced people focuses on “Moment of a Force – 3”.

1. Determine the resultant moment caused by the forces in vector

a) 30i + 40j – 60k Nm

b) 30i – 40j – 60k Nm

c) 30i + 40j + 60k Nm

d) 30i – 40j + 60k Nm

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. Here distance r1 =5j m and r2 = 4i + 5j – 2k m. Thus doing the cross product will give the answer.

2. Which of the following is correct w.r.t the moment (M) of the force (F) acting on the body at a distance L from the axis of the rotation?

a) M=FLsinθ

b) M=FLcosθ

c) M=F.Lsinθ

d) M=FxLsinθ

View Answer

Explanation: The moment of the force about the axis of rotation by the application of the force on the body is given by the cross product of both. If the force not perpendicular to the axis, and making angle θ then cosine form of angle is used. As usually used in the cross product.

3. What does FLsinθ means/represents for the moment (M) of the force (F) acting on the body at a distance L from the axis of the rotation?

a) The direction vector of the moment

b) Unit vector of the moment vector

c) The magnitude of the moment caused by the force on the body

d) The perpendicular distance of the force from the axis of rotation

View Answer

Explanation: The moment of the force about the axis of rotation by the application of the force on the body is given by the cross product of both. If the force not perpendicular to the axis, and making angle θ then cosine form of angle is used. Thus, FLsinθ represents the magnitude of the moment.

4. The basic way of getting the direction of the moment caused by the force is:

a) The use of left hand rule with thumb giving the direction of moment

b) The use of right hand rule with thumb giving the direction of moment

c) The use of right hand rule with forefinger giving the direction of moment

d) The use of left hand rule with forefinger giving the direction of moment

View Answer

Explanation: The basic way of doing so is to use right hand rule and not the left hand rule. The direction of the moment axis is given by the thumb. The direction of the force is given by the fingers. As we place the fingers on the force and curl towards the rotational direction of the body about the axis.

5. If any force is applied in the direction of the positive x-axis, and there are three different point on which the moment of this force is to be calculated. Then if these three points are on the positive side of the y-axis with varying distance, then what will be the direction of the moment caused by the force to the individual point?

a) Towards positive z-axis

b) Towards positive y-axis

c) Towards positive x-axis

d) Towards negative z-axis

View Answer

Explanation: If you will apply he right hand rule on the system given then the right answer is the positive z-axis. Which is because the force is lying on the x-axis and is heading towards the positive infinity of the x-axis. And the points are in the positive y-axis. Apply the rule, and get the direction.

6. If a force applied at any point in its line of action and is still creating the same moment about any fixed point say P, then the force is said to be______________

a) Couple

b) Sliding vector

c) Slider couple

d) Couple slider

View Answer

Explanation: If a force applied at any point in its line of action and is still creating the same moment about any fixed point say P, then the force is said to be sliding vector. This is because the moment of the force which is acting on its line of axis at the point P is same throughout. Whatever be the direction of the distance.

7. : If a force applied at any point in its line of action and is still creating the same moment about any fixed point say P, then the force is said to be sliding vector. What is the name of this property?

a) Associative property

b) Distributive property

c) Negative associative property

d) Principle transmissibility of the force

View Answer

Explanation: If a force applied at any point in its line of action and is still creating the same moment about any fixed point say P, then the force is said to be sliding vector. This is because the moment of the force which is acting on its line of axis at the point P is same throughout. This is known as the principle transmissibility of the force.

8. We can express the force in the Cartesian form.

a) True

b) False

View Answer

Explanation: Yes, we can prepare the moment in the Cartesian form. As the moment in the 3D is the vector. Which can be easily made in the form of Cartesian coordinates. Also it can be seen that the moment is the cross product of the force and the distance, hence the moment is in vector form.

9. M = ∑(rxF) represents what?

a) The total distance of the point of contact of the and the axis of rotation

b) The total moment of the forces

c) The total force acting on the body

d) The equation is wrong, it must be Fxr

View Answer

Explanation: The given equation represents the total moment of the forces which are acting on the body. That is the summation of all the rxF. Where the r is the distance of the axis from the point of action of the force on the body. And thus this is the total summation of the moments of all the forces acting on the body.

10. If a 12m high tree is being pulled by the tractor, by a rope tied over the top. With the tractor at a linear distance of 12m and 4m away perpendicularly from the tree. If the force applied by the tractor is 2KN then what is the moment caused about the roots of the tree?

a) -16.5i+7.51j KNm

b) -16.5i+5.51j KNm

c) -16.5i+5.51j KNmm

d) -16.5i+7.51j KNm

View Answer

Answer: The force developed is 2KN, and the roots are having the coordinate (0, 0, 0). Coordinates of the top of the tree is (0, 0, 12). The tractor’s coordinates are (4, 12, 0). Thus applying the cross product on the force and the distance of the tractor from the roots we get the answer as -16.5i+5.51j KNm.

11. If F = F1+F2, then moment of this force F about a point at a distance r is M=rxF1 + rxF2.

a) True

b) False

View Answer

Explanation: If F = F1+F2, then moment of this force F about a point at a distance r is M=rxF1 + rxF2.

As M = rx(F) = rx(F1 + F2) = rxF1 + rxF2. This is known as the principle of moments. As the force is a vector quantity thus this is the distributive property which we apply to get the answer.

12. Determine the moment of the force about the point X.

a) 11.2 Nm

b) 10 Nm

c) 7Nm

d) 8Nm

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.

13. 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.

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.

15. Determine the magnitude of the resultant moment caused by the forces.

a) 78.1Nm

b) 25Nm

c) 110Nm

d) 80Nm

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. Here distance r1 =5j m and r2 = 4i + 5j – 2k m. Thus doing the cross product will give the answer.

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