# Help Explaining Newton's Third Law

• ediggity
In summary, the conversation discusses a misunderstanding of Newton's Laws, particularly the Third Law and its relationship to the Second Law. The main point is that the Third Law cannot be understood by simply plugging in arbitrary numbers, as it is a general principle that applies to all systems. The conversation also touches on the concept of reference frames and the consistency of the book example with both the Second and Third Laws.
ediggity

## Homework Statement

This isn't a homework question, but I thought this would be the appropriate place for the thread.

I am having trouble explaining Newton's Laws to my friend. His confusion lies in the fact that the second Law F=ma does not equate to the sum of Forces in The third Law.

Ex. A bus, and a mosquito

The mosquito and the bus collide, the Force of the bus on the mosquito is equal to the force of the mosquito on the bus. I tried to tell him they are equal.

He seems to think that if (and these are totally made up numbers)
Mb=100mg
ab=4m/s^2

mm=.2mg
am= .5m/s^2

then Mb(ab) =/= - mm(am), so the third Law is incorrect.

F=ma

F=-F

## The Attempt at a Solution

I tried to explain the conservation of momentum for inelastic and elastic crashes, but he doesn't want to hear it.

Does anyone have any ideas of how I can explain the third law using his not equal example? Thanks for the help in advance.

You can't just make up arbitrary numbers, then use those to try to illustrate general principles, and expect it to work. The fallacy arises from the fact that the numbers are not representative of what would occur in a real example *as a result of Newton's Third Law.*

If a bus (mass M) and a mosquito (mass m) collide, then Newton's Third Law says that

$$F_{bm} = - F_{mb}$$​

Where Fmb is the force of the mosquito on the bus, and vice versa for Fbm. From Newton's second law, the force on the object is equal to its mass times its acceleration. The result is:

$$ma_m = - Ma_b$$​

Therefore, it *must* be true that:

$$\frac{m}{M} = - \frac{a_b}{a_m}$$​

For your made up example, the accelerations and the masses are not related in this way, so of course it doesn't work.

He wants to replace Fbm=-Fmb

with ma

I told him the exact same thing. You can't just make up numbers and plug them into the third Law, and say they don't work.

Would it be correct to say that the third Law in the made up mosquito case is independent of the Second law, because it is taking into account the sum of the Forces?

Well, umm... they are two different statements. One could imagine a universe in which the 3rd law is true but the 2nd is not, or vice-versa. So I guess you could say they're independent.

The 3rd law deals with forces on different objects, while the 2nd law deals with forces on one object.

I also forgot to post his other argument for the Third Law.

I gave him the example for the book on the table, because I figured it would be easiest to understand. However, before I could even explain it he didn't believe it was correct, because the book was not in motion, and they are called Newton's Laws of Motion.

okay, I think I feel your pain

You could tell him there's a whole other set of Newton's Laws of Non-Motion, that just happen to be the same...

diazona said:
okay, I think I feel your pain

I think my next example is going to be explaining the Force from punching myself in the face.

ediggity said:
I also forgot to post his other argument for the Third Law.

I gave him the example for the book on the table, because I figured it would be easiest to understand. However, before I could even explain it he didn't believe it was correct, because the book was not in motion, and they are called Newton's Laws of Motion.

His argument is based on semantics and demonstrates his lack of understanding of what physics is trying to do, which is to determine, what, if any, general laws apply to systems in nature.

If he's really so hung up on the wording, maybe you should ask him the following: is there any reason why Newton's Laws of *Motion* shouldn't tell you when it occurs and when it doesn't? Furthermore, it's only from his point of view that the book is "at rest". From the point of view of an observer in an airplane flying overhead, or an observer on Mars, the book is in motion. There is *no* preferred reference frame that can be used to define an absolute notion of "rest."

As for the book example:

The book example is totally consistent with Newton's Second Law. The net force on the book is zero, therefore it is *not* accelerating.

The book example is totally consistent with Newton's Third Law. The force of the book downward on the table is equal to the force of the table upward on the book.

cepheid said:
His argument is based on semantics and demonstrates his lack of understanding of what physics is trying to do, which is to determine, what, if any, general laws apply to systems in nature.

If he's really so hung up on the wording, maybe you should ask him the following: is there any reason why Newton's Laws of *Motion* shouldn't tell you when it occurs and when it doesn't? Furthermore, it's only from his point of view that the book is "at rest". From the point of view of an observer in an airplane flying overhead, or an observer on Mars, the book is in motion. There is not a preferred reference frame that can be used to define an absolute notion of "rest."

As for the book example:

The book example is totally consistent with Newton's Second Law. The net force on the book is zero, therefore it is *not* accelerating.

The book example is totally consistent with Newton's Third Law. The force of the book downward on the table is equal to the force of the table upward on the book.

Touche

## What is Newton's Third Law?

Newton's Third Law, also known as the Law of Action and Reaction, states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on another object, the second object exerts an equal and opposite force back on the first object.

## Can you explain the concept of "action and reaction" in Newton's Third Law?

The concept of "action and reaction" in Newton's Third Law refers to the equal and opposite forces that are exerted by two objects on each other. This means that the force exerted by the first object is called the action, and the force exerted by the second object is called the reaction.

## How does Newton's Third Law apply to everyday situations?

Newton's Third Law applies to everyday situations in many ways. For example, when you walk, your feet push against the ground with a certain force, and the ground pushes back on your feet with an equal and opposite force. This allows you to move forward. Another example is when you sit on a chair, your weight pushes down on the chair, and the chair pushes back up on you with an equal and opposite force to keep you from falling through.

## Does Newton's Third Law only apply to objects in contact?

No, Newton's Third Law can also apply to objects that are not in contact with each other. For example, when a rocket is launched into space, hot gases are expelled from the rocket with a certain force, and the rocket moves in the opposite direction with an equal and opposite force. This is how rockets are able to propel themselves forward without being in contact with anything.

## What is the significance of Newton's Third Law in physics?

Newton's Third Law is significant in physics because it helps us understand how objects interact with each other and how they move. It also helps us explain many everyday phenomena, such as walking, driving, and even breathing. Additionally, this law is crucial in the study of forces and motion, and it is used in many engineering and design applications, such as building structures and vehicles.

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