What is Newton's Third Law?  

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James Kelley | College Teacher | (Level 1) Educator

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I've always had a little trouble getting my head around this third law, the idea that every action has an equal and opposite reaction. I try to keep two examples in mind:

A book lying on a table is pushing down on the table, because it's being pulled downward by gravity. The table is pushing back, and its capacity for pushing back to the same extent that the book is pushing on it is what keeps the book from crashing through the table.

That example is everyday, but it's not very dynamic. Here's the second example that I like to keep in mind:

If an astronaut in space, just floating there, throws something forward -- say, a spare screwdriver -- the equal and opposing force will propel the astronaut backward. The force of the backward movement of the astronaut will equal the force of the forward throw of the screwdriver. (Why an astronaut would carry a spare screwdrive in space is another problem altogether!)

Those examples have always helped me. The website below, written for students, gives more discussion and examples.

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pohnpei397 | College Teacher | (Level 3) Distinguished Educator

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The way we most often hear Newton's Third Law stated is this: "any action has an equal and opposite reaction."

What this law is saying is that whenever two objects interact, there are two forces acting on those objects.  The forces that are acting have both size and direction.  The sizes of the forces must be the same.  In addition, the directions of the forces must be opposite.

So, for example, a person sitting in a chair is putting downward force on the chair.  But the chair is putting an equal force on the person.

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krishna-agrawala | College Teacher | (Level 3) Valedictorian

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Sir Issac Newton formulated three laws of motions which form the basis for  whole field of classical mechanics. These laws define relationship between forces acting on a body and its motion.

Newtons Third Law of Motion states "To every Action there is always an equal and opposite reaction".

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epollock | (Level 3) Valedictorian

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Newton's Third Law states that for every action (force) in nature there is an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal force on object A. Notice that the forces are exerted on different objects. The third law can be used to explain the generation of lift by a wing and the production of thrust by a jet engine.

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keller888 | High School Teacher | eNotes Newbie

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I have been reading Newton's book, The Principia.  It is this book where he states all his observations about the world.  It is a bridge between physics and philosophy.  In the book, Newton states his Third Law as " For every action, there is an equal and opposite reaction".  It means that when objects interact, there are a pair of forces that act on those objects.  Consider what happens when i baseball player swings a bat and hits a baseball.  The two objects collide and intereact.  The ball goes in the opposite direction and the energy is transfered up along the bat and can be felt by the batter.  Also, think about any type of motorized vehicle.  You step on the gas, the wheels turn in one direction and you move opposite the road.....the road doesn't actually move, but relatively speaking it moves away from the car.  Newton's third law is about the transfer of energy when objects interact.  You can find tons of essays on the web.

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mkcapen1 | Middle School Teacher | (Level 3) Valedictorian

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Newton's Third Law implies that a push or pull on an object will result in the push or pull of another object. Energy does not stand alone, but creates more energy.

The force of one action is equal to the force of another action. For example: As a bird flies it uses its wings to push downward, the force of air reacts by creating an equal force allowing the bird to stay in flight. Another example occurs when a person gets off of a small vessel. As the person applies pressure to the dock, reverse pressure is applied to the vessel which moves it in a backward motion.

"For every action there is an equal and opposite reaction. "

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giorgiana1976 | College Teacher | (Level 3) Valedictorian

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The strongest claims are those with the widest applicability. Newton's Third Law-"for every action there is an opposite and equal reaction" - is a powerful statement because it applies to every action, anywhere, anytime.

But now scientists can test each occurrence of an action, and find a response. So how can they say that the Newton's third law is in some sense true? Of course, many actions were tested, and in each one they could find the opposite reaction of the corresponding action.

But how can we be sure that next time we test Newton's third law, would prove to be valid?

A solution to this problem is to rely on induction. Inductive reasoning starts from the premise that if a situation is valid in all cases observed, then it is valid in all cases. Thus, after a series of experiments which are supporting the Newton's third law, we believe that the law is valid in all cases.

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