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 $4\,\text{kg}$ block, which expression correctly gives the contact force exerted by the $4\,\text{kg}$ block on the $6\,\text{kg}$ block?

The $4\,\text{kg}$ block has the applied force $50\,\text{N}$ forward and contact force backward. Applying Newton's second law to the $4\,\text{kg}$ block gives $50 - N = 4a$, so $N = 50 - 4a$. Option A is correct. Option C is tempting but would represent the net force on the $4\,\text{kg}$ block, not the contact force.