Question

When $2kg$ mass hangs to a spring of length $50cm$, the spring stretches by $2cm$.The mass is pulled down until the length of the spring becomes $60cm$. What is the amount of elastic energy stored in the spring in this condition (in J)

• A. $10$
  
• B. $2$
  
• C. $2.5$
  
• D. $5$
  

Answer 1

The correct option is D $5$

Initial extension$x_1=0.02m$

Final extension $x_2=0.6-0.5=0.1m$

For the initial equilibrium

$\Rightarrow mg=kx$

$\Rightarrow 2\times 10=k\times 0.02$

$\Rightarrow k=1000N/m$

Energy stored =$\frac{1}{2}\times k\times x^2=0.5\times 1000\times {0.1}^2=5J$

Hence correct answer is option $D$