A block of mass 5 kg rides on top of a second block of mass 10 kg. A person attaches a string to the bottom block and pulls the system horizontally across a frictionless surface. Friction between the two blocks keeps the 5 kg block from slipping off. If the coefficient of static friction is 0.350, what maximum force can be exerted by the string on the 10.0 kg block without causing the 5.00 block to slip?
Top Block: mg downwards, normal force up, fs to right.
Bottom Block : mg1 down, mg2 down, normal force up, tension right.
My friend claims that there should also be an fs acting on the Bottom block to the left, but I cant seem to see why. Could someone explain it to me?