Question

A spring is kept compressed by a toy car of mass $150g$. On releasing the car, it moves with a speed of $0.2m{s}^{-1}$. So, the elastic potential energy of the spring is

• A. $3mJ$
• B. $3J$
• C. $1.5mJ$
• D. $4mJ$

Answer 1

Answer: (A)

$3mJ$

Given, $m=150g=150\times {10}^{-3}kg,v=0.2m{s}^{-1}$
On releasing the toy car, the spring tries to come back to its uncompressed state by pushing the car away. Thus the potential energy stored in the spring changes into the kinetic energy of the car.
Elastic potential energy of spring$\left(U\right)$=kinetic energy gained by the cart

So, $U=\frac{1}{2}\times m\times v^2=\frac{1}{2}\times 150\times {10}^{-3}\times {0.2}^2=3\times {10}^{-3}J=3mJ$