A stack of Block A, B of mass 2.0 kg and 4.0 kg respectively are pushed along a smooth horizontal surface by an external force of 12 N. Both blocks move as a stack with the same acceleration. Calculate frictional force between Block A and B.
Fxy denote the contact force that block X exerts on Y.
Block B is accelerated rightward because of the 12 N external force. In the absence of friction, block A would have slid leftward relative to B. This explains why block B exerts a rightward frictional force FBA on block A. Basically it is block B’s attempt to get block A to “stick” with it.
Considering all both blocks as a combined 6 kg mass,
Considering block A by itself as a 2 kg mass,
Alternatively, we can consider blocks B by itself as a 4 kg mass.