Two Marks Questions with Answers

Two Marks Questions with Answers

Q.1

State vector equation for moment of force about a point.

Ans.:

Where is a vector from the point where moment is taken to any point on the line of action of force.

Q.2 

State Varignon's theorem.

Ans.: The algebric sum of moments due to all forces acting on an object about any point is equal to the moment of their resultant about the same point.

Q.3 

Define couple.

Ans.: Two non-collinear parallel forces having same magnitude but opposite direction form a couple.

Q.4

Why the couple moment is said to be a free vector?

Ans.: Couple moment is said to be a free vector as it can be transferred to any point in the plane without causing any change in its effect on the body.

Q.5 

Distinguish between a couple and a moment.

Ans.: Moment represents the turning effect of a force whereas couple consists of two equal and opposite forces separated by some distance. Moment of force varies from point to point but moment of a couple is same about any point in the plane. 

Q.6

What is meant by force-couple system?

Ans.: A system of coplanar non-concurrent force system acting on a rigid body can be replaced by a single resultant force and couple moment at a point known as force-couple system.

Q.7 

State the ways in which the resultant of coplanar non-cone, rent force system can be reduced.

Ans.: A system of non-concurrent forces can be reduced to :

i) A single moment if resultant force is zero.

ii) A force-couple system.

iii) A single resultant force with a specified line of action. The line of action can be specified by the perpendicular distance of resultant force from a given point or x/y intercept on co-ordinate axes or intercept on any inclined axis.

Q.8

Can a coplanar non-concurrent force system with zero resultant force necessarily be in equilibrium?

Ans.: A coplanar non-current force system with zero resultant force is not necessarily in equilibrium as it can have a non-zero resultant moment.

Q.9

State equations to find magnitude and direction of a force if its X, Y and Z components are known.

Ans.: 

Q.10 

When is moment of force zero about a point ?

Ans.: The moment of force about a point is zero if its line of action passes through that point.

Q.11

When is moment of force maximum about a point?

Ans.: Moment of force is maximum about a point when 

i) it is applied at maximum distance from the point and 

ii) it is applied perpendicular to the line joining that point to the point of application of force.

Q.12

When is moment of force zero about a line ?

Ans.: Moment of force about a line is zero when

i) Force is parallel to that line or 

ii) Line of action of force intersects that line.

Q.13

Replace the force 600 N from A as shown Fig. 3.15.1 by equivalent force and couple at B. 

Ans.: As there is only one force, the force at B is

Q.14

Find the resultant of the force systems shown in Fig. 3.15.2.

Ans. :

As R = 0, the resultant is a moment which will be same about any point. Taking moment about O,

M = 300 × 2 + 300 × 5 + 200 × 7

Q.15

Calculate the moment of the 250-N force on the handle of the monkey wrench about the center of the bolt Fig. 3.15.3.

Ans. :

M = 250 sin 15 × 0.03 - 250 cos 15× 0.2

= -46.36 Nm

Q.16

What mechanics concept does the balanced teeter-totter (seesaw) signify ? 

Ans. : The balanced seesaw signifies the principle of moments according to which the total clockwise moment about the pivot equals the total anticlockwise moment.

Q.17

Consider the forces of magnitude P acting on the sides of a regular hexagon with each side of length a. Find the equivalent force system of these forces at point B as shown in the Fig. 3.15.5. 

Ans. : The equivalent force at B = P →. (Except for the horizontal force at the bottom, every other cancels out.)

Q.18

A force F (6i - 4j = 4j - 5k) is acting at a point P whose position vector from the origin O of the co-ordinate axes is 8 mmi + 6 mmj – 4 mmk. Find the moment of this force about the origin.

Ans. :

Q.19

Find the moment of a Force F = (200 i + 100 j) N acting a point 'A' (2i + 3j) m from the origin.

Ans. :