Lami’s Theorm
When 3 coplanar forces acting at a point keeping the point at equilibrium then then ratio of force to
sine of its opp angle remains constant for all forces.
i.e.
P
Q



R
1.
The horizontal force required to pull a body of mass 30gms suspended from a thread, such that the
string makes an angle 300 with vertical is
300
T
1200
F=?
1500
900
mg
F
mg

0
sin150
sin1200
F
0.3

0
 cos 60
sin 600
F
0.3

1
3
2
2
2F 
0.6
3
F
0.3 3 101

3
3
=
3  101  0.0173
= 17.3 × 10-3 N
Alternative method
F = T sin - (1)
mg = Tcos - (2)
Tcos
0
(1), (2) 
0
T
F
Tsin
= 0.0173 = 173 × 10-1N
mg
4.
A body of mass 20kg is suspended by two threads AC and BC shown in fig the tension in AC is
A
B
0
0
45
45
T2
T1
900
1350
C
20kg
20g
T1
20

0
sin135
sin 90
T1 
20
 sin1350
sin 90
=
20
1

1
2
= 10 2 kg wt
A
B
0
0
45
45
T1 = T2 = T
T2
T1
C
20kg
mg
2.
Two strings of length 5, 12cm are connected to a body of wt 100gm and the free ends are attached
to two hooks 13cm apart in same horizontal line. The tension in smaller string.
13 cm
A
B


T2
T1
100
100g
T1
100

sin(90   )
1