# Newton's Third as logical device to make sense of 1st two laws

• \Ron
In summary, the section on Newton's third law in Introduction to Mechanics by Kleppner explains that it is not just a physical principle, but also a logical necessity for the first two laws to make sense. The book presents an experiment where an object in a state of constant velocity suddenly moves, and asks what prevents us from considering the object as completely isolated. It then argues that if there is no third law stating there is always an unequal and opposite reaction to what moves the object, the second law would be meaningless. This concept is questioned as it seems that the second law already accounts for a force that changes the acceleration of the object.

#### \Ron

In Introduction to Mechanics by Kleppner, the section on Newton's third law says that the third law is not only a physical principle but is also a logical necessity for the first two laws to make sense. I don't quite get this.

These statement precedes an experiment regarding an object in a state of constant velocity of zero. Suppose the suddenly moves, the book asks what prevents us from considering that the object is not completely isolated, i.e. we always suppose that there is a force that moves and object. The book then proceeds that if this is true that the object is completely isolated then the second law would be completely meaningless. So there must be a third law which says that there is always an unequal and opposite reaction to what moved the object.

How is this so?

From the 2nd law there must always be a force that moved the object. But isn't it that we can already suppose a force which changes the acceleration of the object based on the Second Law as opposed from the proposition that it is logically required to posit a third law which axiomatizes the existence of a reacting force somewhere in the universe, as a matter of logic.

## 1. What is Newton's Third Law?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This means that when an object exerts a force on another object, the second object will exert an equal force in the opposite direction.

## 2. How does Newton's Third Law relate to the first two laws?

Newton's Third Law provides a logical device to make sense of the first two laws by explaining that forces always occur in pairs. This means that if an object is accelerating, there must be an equal and opposite force acting on it according to Newton's Second Law. And if an object is at rest or moving at a constant velocity, there must be a balanced force acting on it according to Newton's First Law.

## 3. Can you provide an example of Newton's Third Law in action?

One example of Newton's Third Law is when a person jumps off a boat onto a dock. The person exerts a force on the dock, and the dock exerts an equal and opposite force on the person, causing them to move in opposite directions.

## 4. Does Newton's Third Law apply to all types of forces?

Yes, Newton's Third Law applies to all types of forces, including contact forces like pushing and pulling, and non-contact forces like gravity and electromagnetic forces.

## 5. What is the significance of Newton's Third Law in our understanding of the physical world?

Newton's Third Law is significant because it helps us understand that forces always occur in pairs and that every action has an equal and opposite reaction. This is crucial in explaining the motion of objects and the behavior of forces in the physical world.