Laws of Motion - Result Question 20
20. Two blocks $A$ and $B$ of masses $3 m$ and $m$ respectively are connected by a massless and inextensible string. The whole system is suspended by a massless spring as shown in figure. The magnitudes of acceleration of $A$ and B immediately after the string is cut, are respectively:
[2017]
(a) $\frac{g}{3}, g$
(b) $g, g$
(c) $\frac{g}{3}, \frac{g}{3}$
(d) $g, \frac{g}{3}$
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Answer:
Correct Answer: 20. (a)
Solution:
- (a) Before cutting the string
$k x=T+3 mg$
$T=mg$
Force an string,
$\Rightarrow k x=4 mg$
After cutting the string $T=0$ For block A: $3 m a_A=4 mg-3 mg$
$ \begin{aligned} & a_A=\frac{4 m g-3 m g}{3 m}=\frac{g}{3} \\ & a_A=\frac{g}{3} \end{aligned} $
For block B: $m a_B=mg$
$ a_B=g $
