Page 28 - Physics Form 5 KSSM_Neat

1.3 Forces in Equilibrium

Photograph 1.4 shows a Ngajat dancer standing
still for a while during the dance. What is the
relationship between the forces acting on the
dancer? Since the acceleration of the dancer is
KEMENTERIAN PENDIDIKAN MALAYSIA
zero, there is no resultant force acting on him.
Therefore, the forces are said to be in equilibrium.
An object is said to be in equilibrium of
forces when the forces acting on it produce
a zero resultant force. Observe the free body
diagrams for two examples of forces in
equilibrium as shown in Figure 1.23. Photograph 1.4 Ngajat Dance
Info
F = ma
a = 0, F = 0

T = 5 N
P = 3 N

R = 12 N
W = Weight of vase and flowers W = 4 N
R = Normal reaction
W = Weight of lamp
Tiga daya bertindak ke
P = Tension in rope
atas lampu
W = 12 N T = Tension in rope
Resultant force of W and R = 0 Resultant force of W, P and T = 0
(a) Vase of flowers on a table (b) Lamp hung from two strings

Figure 1.23 Examples of forces in equilibrium
In Figure 1.23(a), the vase is in equilibrium of forces. The resultant force of W and R is
zero. In Figure 1.23(b), the lamp also in equilibrium of forces. However, there are three forces
acting on the lamp. This can be represented by a triangle of forces.
A triangle of forces can be drawn to show the equilibrium of
three forces acting on an object. The magnitudes of the three forces are
represented by the lengths of the sides of a triangle and they are drawn W = 4 N T = 5 N
in sequence according to the directions of the forces.

The three forces W, P and T in Figure 1.24 are in equilibrium.
Therefore, the three forces drawn in sequence will form a triangle. P = 3 N
Segi tiga daya
Figure 1.24 Triangle of forces
18 LS 1.3.1