Two bodies A and B of masses 5 kg and 10 kg in contact with each other rest on a table against a rigid wall . The coefficient of friction between the bodies and the table is 0.15. A force of 200 N is applied horizontally to A. What are (a) the reaction of the partition (b) the action-reaction forces between A and B ? What happens when the wall is removed? Does the answer to (b) change, when the bodies are in motion? Ignore the difference between `mu_s` and `mu_k`

Explanation

We assume perfect contact between bodies A and B and the rigid partition. In that case, the self-adjusting normal force on B by the partition (reaction) equals 200 N. There is no impending motion and no friction. The action-reaction forces between A and B are also 200 N. When the partition is removed, kinetic friction comes into play.
Acceleration of `A+B = [ 200 - (150 xx 0.15 )]//15 = 11.8 ms^(-2)`
Friction on A` = 0.15 xx 50 = 7.5 N`
`200 - 7.5 - F_(AB) = 5 xx 11.8`
`F_(AB) = 1.3 xx 10^2 N` , opposite to motion
`F_(BA) = 1.3 xx 10^2 N` , in the direction of motion