Engineering Mechanics

Chapter 2 : Composition and Resolution of Forces „„„„„ 19

Fig. 2.3.

Example 2.5. A triangle ABC has its side AB = 40 mm along positive x-axis and side
BC = 30 mm along positive y-axis. Three forces of 40 N, 50 N and 30 N act along the sides AB, BC
and CA respectively. Determine magnitude of the resultant of such a system of forces.

Solution. The system of given forces is shown in Fig. 2.3.
From the geometry of the figure, we find that the triangle ABC is a right angled triangle, in
which the *side AC = 50 mm. Therefore

30

sin 0.6

50

θ= =

and
40
cos 0.8
50

θ= =

Resolving all the forces horizontally (i.e., along AB),

∑H = 40 – 30 cos θ

= 40 – (30 × 0.8) = 16 N

and now resolving all the forces vertically (i.e., along BC)

∑V = 50 – 30 sin θ

= 50 – (30 × 0.6) = 32 N

We know that magnitude of the resultant force,

RH V= (∑ )22 22+(∑ ) =+=(16) (32) 35.8 N Ans.

Example 2.6. A system of forces are acting at the corners of a rectangular block as shown
in Fig. 2.4.

Fig. 2.4.
Determine the magnitude and direction of the resultant force.
Solution. Given : System of forces
Magnitude of the resultant force

Resolving forces horizontally,

∑H = 25 – 20 = 5 kN

and now resolving the forces vertically

∑V = (–50) + (–35) = – 85 kN

∴ Magnitude of the resultant force

RH V= (∑ )^2222 +(∑ ) =+(5) (–85) = 85.15 kN Ans.

* Since the side AB is along x-axis, and the side BC is along y-axis, there fore it is a right-angled triangle.

Now in triangle ABC,

AC=+= +=AB22 2 2BC (40) (30) 50 mm