Here, mass of body A, $m_1 = 5$ kg, mass of body B, $m_2 = 10$ kgCoefficient of friction between the bodies and the table, $m = 0.15$.Horizontal force applied on body A, $P = 200$ N(a) Reaction of partition = ?Let f = force of limitingfriction acting to the left, then $f = \mu R = \mu (m_1 + m_2) g$ $[\because R = (m_1 + m_2) g]$ $= 0.15 (5 + 10) \times 10 = 22.5 \text{ N}$∴ If F′ = Net force to the right exerted on the partition, then$F' = P - f = 200 - 22.5 = 177.5 \text{ N}$∴ According to Newton’s3rd law of motionReaction of the partition = 177.5 N to the left(b) Action-reaction forces between A and B = ?Let $f_1 =$ force of limiting friction acting on body A.$f_1 = \mu R_1 = \mu m_1 g$ (∴ here $R_1 = m_1 g$) $= 0.15 \times 5 \times 10 = 7.5 \text{ N}$IfP′ the net force exerted by body A on body B.Then $P' = P - f_1 = 200 - 7.5 = 192.5 \text{ N}$i.e. action of A on B = P′ = 192.65 N towards right∴ According to Newton’s 3rd law of motion,Reaction of B on A = 192.5 N towards left.Note :If we assume perfect contact between bodies A and B and the rigid partition, then the self adjusting normal force on B by the partition (reaction) equals the applied force i.e. 200 N. There is no impending motion and no friction. The action-reaction forcesbetween A and B are also 200 N. When the partition is removed : When the partition is removed, the kinetic friction comes into play, the masses move together as a system of two bodies under the action of net force F′ given by $F' = P - f = 200 - 22.5 = 177.5 \text{ N}$ If a = acceleration produced in the system, then $a = \frac{F'}{m_1 + m_2}$ $= \frac{177.5}{5+10} = \frac{177.5}{15} = 11.83 \text{ m s}^{-2}$Does the answer to (b) change if $m_1$ and $m_2$ are in motion?Yes, when the bodies arein motion, then the answer to (b) changes and can be proved as follows :When the bodies are moving, the force exerted by A on B is given by$F_{BA} =$ P′ – force required to produce an acceleration of 11.83 m s$^{-2}$ in body A alone $= P - f_1 - m_1 a$ $= 200 - 7.5 - 5 \times 11.83$$= 200 - 7.5 - 59.30$ $= 192.5 - 59.15 = 133.35 \text{ N}$Action of A on B when partition is removed = 133.35 N ∴ reaction of B on A, when partition is removed = 133.35 N to the left.