The answer is more than one. The two laws that come into play are Newton's first and second laws. Newton's first law is best known as the law of inertia. It states that an object in motion will stay in motion and an object at rest will stay at rest unless acted on by an outside force. Newton's second law relates the acceleration of an object to the mass and the forces acting on it with the equation . An object won't move or stop moving unless the forces acting on it are imbalanced. In the case of the milk jug, it remained at rest until an outside force, your hand, acted upon it, demonstrating Newton's first law. The second law is applicable because the amount of acceleration of the two milk jugs is inversely related to their mass. It takes a much stronger force to move the 1000L milk jug than it does the 1L milk jug. The acceleration is also much smaller for the two objects when the same force is applied because one weighs so much more than the other. You can visualize this by rearranging the second law equation:  . It is also beneficial to think in terms of two objects. Which is will accelerate more when you try and move it with the same force, a tennis ball or an elephant?

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

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

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.

This is an example of an inelastic collision, as the two cars stick together after colliding. We can assume momentum is conserved.

To make the equation easier, let's call the first car "1" and the second car "2."

Using conservation of momentum and the equation for momentum, , we can set up the following equation.

Since the cars stick together, they will have the same final velocity. We know the second car starts at rest, and the velocity of the first car is given. Plug in these values and solve for the final velocity.

This is an example of an elastic collision. We start with two masses and end with two masses with no loss of energy. 

We can use the law of conservation of momentum to equate the initial and final terms.

Plug in the given values and solve for .