# Engineering Mechanics Questions and Answers – Moment of a Force – 2

This set of Engineering Mechanics Questions and Answers for Experienced people focuses on “Moment of a Force – 2”.

1. Find the moment of the force about the point R.

a) 5000Nm
b) 5550Nm
c) 6000Nm
d) 7000Nm

Explanation: As we know that the moment is the cross product of the distance and the force we will try to apply the same here. We see that the perpendicular distance is 5m. Thus we get the distance. And hence multiply it with the force, 5x2000xsin30. Because the force component perpendicular to the distance needs to be taken.

2. How far from the shaft at P a 200N vertical force must act so as to create the same moment as produced by the 75N, at P?

a) 1.2m
b) 1.125m
c) 0.6m
d) 0m

Explanation: As we know that the moment is the cross product of the distance and the force we will try to apply the same here. So let the distance be y, then the force of 200N will act downward. The distance perpendicular is xcos60. Thus equating the 112.5 to 200ycos60.

3. Find the moment along the point at the mid of the line SR.

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a) 1200Nm
b) 600Nm
c) 0Nm
d) 1400Nm

Explanation: As we know that the moment is the cross product of the distance and the force we will try to apply the same here. But here we have a number of forces components acting on the T point. So by adding all the moments caused by all the forces will give us the value as 600Nm.

4. Which of the following is true?
a) If we are considering the clockwise direction to be positive then the rotation along the counter clockwise direction is negative
b) If we are considering the clockwise direction to be negative then the rotation along the counter clockwise direction is also negative as no such assumptions can be taken
c) If we are considering the clockwise direction to be positive then the rotation along the counter clockwise direction is also positive as no such assumptions can be taken
d) If we are considering the clockwise direction to be negative then the rotation along the counter clockwise direction is neither positive nor negative as we only pre assumed the clockwise direction

Explanation: We can take such assumptions. We are free to do so. We just need to specify if clockwise is positive then the counter clockwise is to be negative. Also, such an assumption is very useful in 2D calculation of the moments caused by various forces on the body.

5. The moment is the cross product of which two vectors?

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.

6. The ___________ forces do not cause the rotation.
a) Non-concurrent
b) Concurrent
c) Parallel
d) Non-Parallel

Explanation: The concurrent forces are the which are somewhere touching the axis of rotation. If any of the force is touching that axis, that force is not considered or is insufficient to cause a rotation. If a force is concurrent then the perpendicular distance of the force from the line of an axis is zero, thus no rotation. As we know rotation is caused by a moment.

7. Choose the correct one.
a) In a coplanar system the moment of the force is chose about a point instead of axis
b) In a coplanar system the moment of the force is chose about an axis instead of a point
c) In a 3D system the moment of the force is chose about a point instead of axis
d) In a 3D system the moment of the force is chose about an axis which is perpendicular to the direction of the force

Explanation: The coplanar system means the consideration is with the 2D only. No 3D. Thus if there is any moment needed to be considered we need to take it along the point and not along the axis. But talking about the axis, there might be no axis in the 2D, as it is only in the 3D.

8. Determine the moment about the point S.

a) 0Nm
b) 350Nm
c) 100Nm
d) 200Nm

Explanation: As we know that the moment is the cross product of the distance and the force we will try to apply the same here. We see that the perpendicular distance is 2m. Thus we get the distance. And hence multiply it with the force, 2×50.

9. Moments can be added like scalars that is it can be added algebraically with proper signs.
a) True
b) False

Explanation: The moments in the 2D can be added algebraically just like the scalar quantities. But the need is that the signs must be taken into consideration. As if clockwise is positive, so anti clockwise is negative. And the result can be negative too. That is a counter clockwise direction.

10. If you are opening the door, then the force you apply is greater if you open it from the part closer to the hinges.
a) True
b) False

Explanation: As the moment is the product of the force and the perpendicular distance of the point of action and the axis, thus more the distance more will be the moment. And hence to overcome that you have to apply more energy. Thus the answer.

11. Which of the following is correct in the determination of the moment direction by curling of wrist?
a) The thumb represents the direction of the force
b) The thumb represents the direction of the moment
c) The fingers represent the direction of the force
d) The direction in which you curl your wrist is towards the direction of the distance from point of contact of force to the axis of rotation.

Explanation: The curled hand represents a various thing. 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.

12. Find the moment along R.

a) 1200Nm
b) 600Nm
c) 0Nm
d) 1400Nm

Explanation: As we know that the moment is the cross product of the distance and the force we will try to apply the same here. But here we have a number of forces components acting on the T point. So by adding all the moments caused by all the forces will give us the value as 0Nm.

13. For the below diagram determine the magnitude of the horizontal force applied at R which creates the same moment at P as by 75N.

a) 110N
b) 43.1N
c) 60N
d) 0N

Explanation: As we know that the moment is the cross product of the distance and the force we will try to apply the same here. We see that the perpendicular distance is 3sin60. Thus we get the distance. And hence multiply it with the force, moment = 112.5Nm. Because the force component perpendicular to the distance needs to be taken.

14. Which of the following is true?
a) Total moment of various forces acting on the body is the vector sum of all moments in 3D
b) Total moment of various forces acting on the body is the algebraic sum of all moments in 3D
c) Total moment of various forces acting on the body is always zero in any dimension
d) Total moment of various forces acting on the body is the vector sum of all moments which is perpendicular to each other forces whatever be the dimensions

Explanation: When we consider the dimensions we need to be careful. The moment is the vector quantity. Thus the value of the total moment caused by various forces acting on the body is the vector sum of all the vectors. Also, the moments are not perpendicular to each other unless it is specified.

15. The moment axis is in the direction parallel to the plane of the force and the distance.
a) True
b) False

Explanation: The moment axis is always perpendicular to the planes of the force and the distance of the axis and the point of action of the force on the body. This means that the moment is the cross product of the force and the distance between the axis and the point of action of the force.

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