A force of $50\,\text{N}$ acts on two connected blocks of masses $4\,\text{kg}$ and $6\,\text{kg}$ on a frictionless surface. If the force is applied to the $6\,\text{kg}$ block, which equation correctly represents the motion of the $6\,\text{kg}$ block?

When the $6\,\text{kg}$ block is pushed, the applied force $50\,\text{N}$ acts forward and the contact force $N$ from the other block acts backward. So by Newton's second law, $50 - N = 6a$. Option A is correct. Option C ignores the applied force, and D uses the wrong sign for the contact force.