# Engineering Mechanics Questions and Answers – Characteristics of Dry Friction – 2

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This set of Engineering Mechanics online quiz focuses on “Characteristics of Dry Friction – 2”.

1. For making the equilibrium equations the normal forces acts in which direction in the free body diagrams?
a) Vertically Upward
b) Vertically Downward
c) Horizontally Right
d) Horizontally Left

Explanation: As the loads are acting in the downward direction. Thus to make the forces balance, the normal forces act in the vertically upward direction. As we know that when there is no lubricating fluid present between the surfaces in contact, the dry friction occurs. This friction magnitude is taken out from these normal forces.

2. Which one is not the condition for the equilibrium in free body diagram for calculation of the normal forces, consider all forces to be straight and linear?
a) ∑Fx=0
b) ∑Fy=0
c) ∑Fz=0
d) ∑F≠0

Explanation: For the equilibrium in the three dimensional system of axis we have all the conditions true as, ∑Fx=0, ∑Fy=0 and ∑Fz=0. Also, we have the summation of the forces equal to zero. Which is not a non-zero value.

3. We show the net forces by the help of __________ forces.
a) Rotational
b) Linear
c) Helical
d) Resultants

Explanation: The net forces acting on the body is shown with the help of the resultant forces. This is because the resultant forces have the sum of all the forces which are acting in the direction which is same. Thus the resultant forces are used to show the net forces acting in the body.

4. There are main two types of forces which are being stated in the free body diagram, they are generally the resultant forces which are being acted over the body. Which are they?
a) Normal and Frictional
b) Normal and Vertical
c) Vertical and Frictional
d) Normal and Fractional

Explanation: The net forces acting on the body is shown with the help of the resultant forces. There are two types, first the frictional and the second is the normal. This is because the resultant forces have the sum of all the forces which are acting in the direction which is same.

5. In the problems regarding the friction and normal force calculations, there is no scope of the two force and three force systems.
a) True
b) False

Explanation: The two and three force member system can be acted upon the body even if there are friction calculations involved. The main thing is that the frictional forces are the phenomena in which there is a resistance between the surfaces. And the resistance magnitude is given by those calculations. So if two or three system does comes, it has no effect.

6. There is the application of the Newton’s third law of motion in the free body diagrams of the friction calculations.
a) True
b) False

Explanation: There is some calculations involved in which we have to make use of Newton’s third law of motion. The requirement of the third law is important in the equilibrium of the body. The body particles are in the equilibrium and are thus facing the forces and to be in the equilibrium they also react and apply the opposite force and thus the third law of newton.

7. As we are using vector math to solve for the unknown forces in the free body diagrams, what is k.i?
a) 0
b) 1
c) -1
d) ∞

Explanation: As the dot product of only the same Cartesian component is unity, i.e. i.i = 1 and j.j =1, rest all remaining dot product will give 0(i.j = 0 and j.k = 0). Cross product of the same plane vectors always give zero. This is generally done because the normal forces are perpendicular to the surfaces of action of the friction.

8. The three force system can also be in the equilibrium if:
a) All the forces are parallel to each other heading towards the same direction
b) The force components cancel each other
c) The forces are very small in magnitude
d) The forces are very huge in magnitude

Explanation: Even if the forces components cancels each other than the body is said to be in equilibrium. Also, it should satisfy whether the moments caused by the forces are equal to zero or not. This means that we need to take components of the normal and frictional forces so as to make the desired calculations.

9. The normal forces and the forces of friction are collinear.
a) True
b) False

Explanation: They are not collinear. They are just acting perpendicular or parallel to the surface of action. Thus they are not even on the same plane. So as to make the calculations proper we need to see the same and then apply the equations.

10. Determine the horizontal force acting in the given figure. a) 80cos30
b) 60cos30
c) 40cos30
d) 8cos30

Explanation: The net forces acting on the body is shown by the help of the resultant forces. There are two types, first the frictional and the second is the normal. This is because the resultant forces have the sum of all the forces which are acting on the direction which is same.

11. Determine the vertical force acting in the given figure. a) 236N
b) 600n
c) 403N
d) 830N

Explanation: The net forces acting on the body is shown with the help of the resultant forces. There are two types, first the frictional and the second is normal. This is because the resultant forces have the sum of all the forces which are acting in the direction which is same.

Sanfoundry Global Education & Learning Series – Engineering Mechanics.

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