Let there be a block of mass $m$ on a frictionless surface. A boy of mass $M$ exerts an invariant force $F$ on the block & does positive work on the block by displacing it by $d$. By Law of Conservation of Linear Momentum , the block will also exert same force $F$ on the boy but in the opposite direction and will do positive work by displacing the boy by $ \frac{m}{M} .d$ .
Now one thing that confuses me: From where does the block get the energy required to do positive work on the boy? I am sure it is definitely not its kinetic energy otherwise, the velocity of the block would decrease. The boy uses the energy he got from his food to do positive work on the block. But from where does the block get the energy for doing positive work on the boy?? Please help.