Question

Two tennis balls, each of mass $60g$, are attached with a massless rubber thread and held in the vertical position as shown in the figure. In this position, the length of the rubber thread is $40cm$ and it is unstretched. The upper ball is slowly raised vertically upward, until the lower ball just becomes unsupported by the ground. At this time the length of the thread is $1m$. The rubber thread exerts a force which is proportional to its extension.

How much work is done (in $J$) by the external agent when the upper ball was raised?

Answer 1

Answer: 0.54

$kx=0.06g$
$k\times 0.6=0.6$
$k=1N/m$
$w_{\text{ext }}+w_g+w_{\text{string }}=\Delta K=0$
$w_{\text{ext }}-0.06\times g\times 0.6+\frac{1}{2}\times 1\times \left(0^2-0.6^2\right)=0$
$w_{\text{ext }}=0.54J$