# Why doesn't Newtons third law also apply to the frame?

• Callista
In summary, the force exerted by the locomotive on the wall was the same in magnitude as the force exerted by the wall on the locomotive. The wall was able to break because not all of the forces applied by the train were transferred to the wall-frame interface.
Callista

## Homework Statement

A locomotive has broken through the wall of a train station. During the collision, what can be said about the force exerted by the locomotive on the wall?
a)The force exerted by the locomotive on the wall was less than the force exerted by the wall on the locomotive.
b) The force exerted by the locomotive on the wall was the same in magnitude as the force exerted by the wall on the locomotive.
c) The force exerted by the locomotive on the wall was larger than the force the wall could exert on the locomotive.
d) The wall cannot be said to "exert" a force; after all, it broke.

NA?

## The Attempt at a Solution

I understand that the answer must be b, due to Newton's 3rd law. However, what I don't understand is my textbooks explanation for why the wall gives out.

"Answer (b). Newton’s 3rd law describes all objects, breaking or whole. The force that the locomotive exerted on the wall is the same as that exerted by the wall on the locomotive. The framing around the wall could not exert so strong a force on the section of the wall that broke out."

According to Newtons 3rd law there must be an equal and opposite reaction between the wall and the train. So why isn't there a equal and opposite reaction between the wall and the frame? I feel like there must be an unequal force somewhere to cause the wall to break, but I can't understand how this could be possible according to Newtons 3rd law.

The opposing forces on the train are the force from the wall frame (delivered via the wall), the force required to break the wall into pieces, the force required to accelerate the wall pieces, plus some heat in the form of speeding up the molecules in the wall pieces. The more you dig into where the forces go the more complicated it is. For example what about the force required to move the air that results in the sound of the crash, etc. So, everyone can't help but simplify the description to just say the force was in the wall.
There is an equal and opposite force exchange in the wall - frame interface, but it didn't include all of the forces applied by the train. Some of those forces did other things.

Callista
Callista said:
According to Newtons 3rd law there must be an equal and opposite reaction between the wall and the train. So why isn't there a equal and opposite reaction between the wall and the frame?
There is.

I feel like there must be an unequal force somewhere to cause the wall to break, but I can't understand how this could be possible according to Newtons 3rd law.
The mistake in your reasoning is assuming that the force exerted by the train on the wall is equal in magnitude to the force exerted by the frame on the wall. Those are two different interactions, so Newton's third law says nothing about how they compare.

Chestermiller

## 1. Why doesn't Newton's third law also apply to the frame?

Newton's third law states that for every action, there is an equal and opposite reaction. However, this law applies specifically to objects or bodies in motion, not to frames or reference points.

## 2. Can Newton's third law be applied to the frame in certain situations?

In some situations, the frame or reference point can be considered an object in motion and Newton's third law can be applied. For example, if the frame is accelerating, then the forces acting on it can be analyzed using Newton's third law.

## 3. How does the concept of inertia relate to Newton's third law and frames?

Inertia is the tendency of an object to resist changes in its state of motion. Newton's third law applies to objects in motion because they have inertia. Frames, on the other hand, are simply reference points and do not have inertia, so Newton's third law does not apply to them.

## 4. Is there a similar law to Newton's third law that applies to frames?

No, there is not a specific law that applies to frames in the same way that Newton's third law applies to objects in motion. However, frames can be used to analyze and understand the application of Newton's third law to objects in motion.

## 5. Why is it important to understand the limitations of Newton's third law when studying frames?

It is important to understand that Newton's third law does not apply to frames because it can lead to misunderstandings and incorrect analyses of forces and motion. By understanding the limitations of the law, scientists can accurately apply it to objects in motion while using frames as reference points.

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