From hook's law
F = -kx
∵ F = mg
when 100 g then f = 100 g
100 g = k(60 - l) ......(1)
When 120 g then f = 120 g
120 g = k(70 - l) ......(2)
Dividing equation (1)/(2)
\(\frac{100\,g}{120\,g} = \frac{60 - l}{70 - l}\)
\(\frac{10}{12} = \frac{60 - l}{70 - l}\)
700 - 10 l = 720 - 12 l
12 l - 10 l = 720 - 700
2 l = 20
l = 10 cm
F = kx
0.1 × 10 = k × 0.6
k = 10/6
k = 1.66 N/m = \(\frac{1.66 \times 10^5}{100}\)
k = 1.66 × 103 dyne/cm
F = k(74 - 10)
F = 1.66 × 103 × 64
F = 106.24 × 103 dyne
F = \(\frac{106.24 \times 10^3}{10^5}\)
F = 106.24 × 10-2 N
F = \(\frac{106.24 \times 10^3}{10}\times1000\)
F = 106.24 gram