Question

When a 20 g mass hangs attached to one end of a light spring of length 10 cm, the spring stretches by 2 cm. The mass is pulled down until the total length of the spring is 14 cm. The elastic energy, (in Joule) stored in the spring is

• A. $4\times {10}^{-2}$
• B. $4\times {10}^{-3}$
• C. $8\times {10}^{-2}$
• D. $8\times {10}^{-3}$

Answer 1

Answer: (D)

$8\times {10}^{-3}$

spring force $F=kx$
$20\times {10}^{-3}\times 10=k\times 2\times {10}^{-2}$
$k=10N/m$
Energy stored in spring $=\frac{1}{2}k{x}^2$
$=\frac{1}{2}\times 10\times (4\times {10}^{-2}{)}^2$
$=8\times {10}^{-3}J$.