Three blocks A, B and C, of masses 4 kg , 2 kg 1 kg and respectively, are in contact on a frictionless…

Three blocks A, B and C, of masses $4 \mathrm{kg}, 2 \mathrm{kg}$ $1 \mathrm{kg}$ and respectively, are in contact on a frictionless surface, as shown. If a force of $14 \mathrm{N}$  is applied on the$4 \mathrm{kg}$ block, then the contact force between A and B is:

1. $2 \mathrm{N}$
2. $6 \mathrm{N}$
3. $8 \mathrm{N}$
4. $18 \mathrm{N}$

Solution

$\begin{aligned} & F=M a=(M_{a}+M_{b}+M_{c}) a \\ & 14=(4+2+1) a=7 a \\ & a=2 \end{aligned}$ Let $F^{\prime}$ be the contact force between $A$ and $B$, $\begin{aligned} & F-F^{\prime}=4 a \\ & 14-F^{\prime}=4 a=4 \times 2 \\ & F^{\prime}=6 N\end{aligned}$

Asked in: NEET 2015 (Phase 1)

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