Newton's third law states that when one body exerts a force on another body, the second body exerts a force equal in magnitude, but opposite in direction, on the first body.

Mathematically, this process can be written as:

Since the rock exerts  of force on the window, then the window must exert  of force on the rock.

Newton's third law states that when one object exerts a force on another object, that second object exerts a force of equal magnitude, but opposite in direction on the first.

That means that:

Using the value from the question, we can find the force of the ground on the boy.

According to Newton’s 3rd Law of Motion, the force that is exert by object A onto object B is equal in magnitude and opposite in direction to the force that is exerted by object B onto object A.  It is not dependent on the mass or motion of the object.

There is a common misconception that the force that Earth pulls on the object is balanced by a reaction force of the table pushing up on the object.  Though it is true that the magnitude between these forces are the same, they are not considered action reaction pairs.  

According to Newton’s 3rd law the force with which object A acts on object B is equal and opposite to the force that object B acts on object A.  In this case the initial force is the Earth pulling on the object.  Therefore the pair would be the object pulling on the Earth.

If we consider the table we are adding a third object to the mix, which cannot be an action reaction pair.  Therefore the normal force and the force of gravity are never considered action reaction pairs as they are two different forces acting on the same object.

There is a common misconception that the force that Earth pulls on the object is balanced by a reaction force of the string pulling upward on the object.  Though it is true that the magnitude between these forces are the same, they are not considered action reaction pairs.  

According to Newton’s 3rd law the force with which object A acts on object B is equal and opposite to the force that object B acts on object A.  In this case the initial force is the Earth pulling on the object.  Therefore the pair would be the object pulling on the Earth.

If we consider the table we are adding a third object to the mix, which cannot be an action reaction pair.  Therefore the tension force and the force of gravity are never considered action reaction pairs as they are two different forces acting on the same object. 

For this problem, we'll use Newton's third law, which states that for every force there will be another force equal in magnitude, but opposite in direction.

This means that the force of the first skater on the second will be equal in magnitude, but opposite in direction:

Use Newton's second law to expand this equation.

We are given the mass of each skater and the acceleration of the first. Using these values, we can solve for the acceleration of the second.

From here, we need to isolate the acceleration of the second skater.

Notice that the acceleration of the second skater is negative. Since she is moving in the opposite direction of the first skater, one acceleration will be positive while the other will be negative as acceleration is a vector.