Answer:
The force of attraction between the molecules of the same substance
Explanation:
There are many forms of energy, but they can all be put into two categories: kinetic and potential. Kinetic energy is motion––of waves, electrons, atoms, molecules, substances, and objects. Potential energy is stored energy and the energy of position––gravitational energy
Newton's second law states that the resultant of the forces applied to an object is equal to the product between the object's mass and its acceleration:

where in our problem, m is the mass the (child+cart) and a is the acceleration of the system.
We are only concerned about what it happens on the horizontal axis, so there are two forces acting on the cart+child system: the force F of the man pushing it, and the frictional force

acting in the opposite direction. So Newton's second law can be rewritten as

or

since the frictional force is 15 N and we want to achieve an acceleration of

, we can substitute these values to find what is the force the man needs: