Laws of Motion - Result Question 25
25. Three blocks with masses $m, 2 m$ and $3 m$ are connected by strings as shown in the figure. After an upward force F is applied on block $m$, the masses move upward at constant speed $v$. What is the net force on the block of mass $2 m$ ? ( $g$ is the acceleration due to gravity)
[2013]
(a) $2 mg$
(b) $3 mg$
(c) $6 mg$
(d) zero
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Answer:
Correct Answer: 25. (d)
Solution:
- (d)
From figure
$F=6 mg$,
As speed is constant, acceleration $a=0$
$\therefore 6 mg=6 ma=0, F=6 mg$
$\therefore T=5 mg, T^{\prime}=3 mg$
$T^{\prime \prime}=0$
$F _{\text{net }}$ on block of mass $2 m$
$=T-T^{\prime}-2 mg=0$
As all blocks are moving with constant speed, then acceleration is zero, so force is zero.
$\because v=$ constant
so, $a=0$, Hence, $F _{\text{net }}=m a=0$
