Let's say that (from left to right) we have two cubes A and B in contact on a plane.  If a force F is applied (from left to right) to A the fact that A moves towards B applies a force S to B that has a reaction $R_s$ on A.  So the equations of the total forces are: for $A: m_a \times a_a = F - R_s$;    for $B: m_b \times a_b = S$  In this situation I can't get why A and B would have the same acceleration $a_a = a_b$ that keeps them in contact. Actually why B couldn't gain an higher acceleration than A?  PS: sorry for the bad english, I'm not a native speaker, if something isn't clear I'll try to explain myself again.
Sorry for posting such a simple problem but I'm having trouble understanding this concept.  In the picture below, we have a force, $ \vec F_{app}$ acting on two objects, A and B.  Mass of block A: $ M_a = 4kg $  Mass of block B: $ M_b = 6kg$  The acceleration of the two blocks can be found using F=ma  $$ 20N = 10kg * a $$ $$ a = 2m/s^2 $$  If we find the force Fba on block B from block A we get:  $$ \vec F_{ba} = 6kg * 2m/s^2 $$ $$ \vec F_{ba} = 12N $$  What I am confused about here is why don't we say that the total force $ \vec F_{app} $ applied onto block A is also applied to block B (20N). Is the force somehow distributed into the blocks?
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