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:
Answer:
Experiment plays many roles in science. One of its important roles is to test theories and to provide the basis for scientific knowledge. ... Experiment can provide hints toward the structure or mathematical form of a theory and it can provide evidence for the existence of the entities involved in our theories.
Explanation:
Given: uniform electric field E= 82.0 kN/C.
a) charge density σ =ε_0 E.
therefore, 
b)Total charge on each face = σA
q=σA
