# Where does the block get the energy from?

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.

• In this case I think the block do so for its inertia. According to 1st law of motion, when we are going to change the state of motion( i.e. rest or moving) we have to apply force on it. In this case when the boy exerts a force on a block, the block also give the same and opposite forces on the boy which is the reaction force and its appears due to the Inertia property of the matter. This reaction force does a positive work on the boy. – Rajesh Sardar Nov 12 '14 at 3:47
• Have I written anything different. Yes the reactive force of the block does positive work on the boy. But from where does it get the energy for doing this work?? – user36790 Nov 12 '14 at 4:02