Engineering Mechanics

Chapter 2 : Composition and Resolution of Forces „„„„„ 23

EXERCISE 2.1

1. Find the resultant of two forces equal to 50 N and 30 N acting at an angle of 60°.
   [Ans. 70 N ; 21.8°]


2. Two forces of 80 N and 70 N act simultaneously at a point. Find the resultant force, if the angle
   between them is 150°. [Ans. 106.3 N ; 61°]


3. Find the resultant of two forces 130 N and 110 N respectively, acting at an angle whose tangent
   is 12/5. [Ans. 185.7 N ; 30.5°]


4. A push of 180 N and pull of 350 N act simultaneously at a point. Find the resultant of the
   forces, if the angle between them be 135°. [Ans. 494 N ; 30°]


5. Find the angle between two equal forces P, when their resultant is equal to (i) P and (ii) P/2.
   [Ans. 120° N ; 151°]
   Hint. When resultant is equal to P, then

PPP PP=++^22 2. cosθ=+ θP2 2cos
∴ 2 cos θ = – 1 or cos θ = – 0.5 or θ = 120° Ans.
When resultant is equal to P/2, then

(^) 0.5PPP PP=++^22 2. cosθ=+ θP 2 2 cos
∴ 2 cos θ = – 1.75 or cos θ = –0.875 or θ = 151° Ans.

6. The resultant of two forces P and Q is R. If Q is doubled, the new resultant is perpendicular to
   P. Prove that Q = R.
   Hint. In first case, RPQ PQ=++^22 2cosθ

In second case,
(2 ) sin
tan 90
(2 ) cos

Q

PQ

θ

°=

+θ

Since tan 90° = ∞, therefore P + 2Q cos θ = 0

2.15.LAWS FOR THE RESULTANT FORCE

The resultant force, of a given system of forces, may also be found out by the following laws :

1. Triangle law of forces. 2. Polygon law of forces.

2.16.TRIANGLE LAW OF FORCES

It states, “If two forces acting simultaneously on a particle, be represented in magnitude and
direction by the two sides of a triangle, taken in order ; their resultant may be represented in magnitude
and direction by the third side of the triangle, taken in opposite order.”

2.17.POLYGON LAW OF FORCES

It is an extension of Triangle Law of Forces for more than two forces, which states, “If a
number of forces acting simultaneously on a particle, be represented in magnitude and direction, by
the sides of a polygon taken in order ; then the resultant of all these forces may be represented, in
magnitude and direction, by the closing side of the polygon, taken in opposite order.”

2.18.GRAPHICAL (VECTOR) METHOD FOR THE RESULTANT FORCE

It is another name for finding out the magnitude and direction of the resultant force by the
polygon law of forces. It is done as discussed below :

1. Construction of space diagram (position diagram). It means the construction of a diagram
   showing the various forces (or loads) alongwith their magnitude and lines of action.