A question about opposite and equal reactions

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Discussion Overview

The discussion revolves around Newton's third law of motion, specifically the interpretation of "for every action there is an opposite and equal reaction." Participants explore scenarios involving a rock being thrown at a brick wall versus a glass window, questioning the applicability of the law in these contexts. The conversation includes theoretical considerations, examples, and challenges to the law's validity based on observed outcomes.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory
  • Mathematical reasoning

Main Points Raised

  • Some participants argue that when a rock is thrown at a glass window, it does not exhibit an opposite and equal reaction as expected by Newton's third law, since the rock continues in the same direction after breaking the window.
  • Others clarify that the window does exert an equal and opposite force on the rock, but the force may not be sufficient to reverse the rock's momentum, leading to the window shattering instead.
  • A participant explains that the interaction between the rock and the window is different from that with a wall, emphasizing that the forces involved depend on the materials' properties and the nature of the interaction.
  • Some participants provide mathematical examples to illustrate the relationship between force, momentum, and acceleration, suggesting that the change in momentum of the rock and the window is equal and opposite, despite the window breaking.
  • There is a discussion about the difference between force and momentum, with some participants asserting that they are related but distinct concepts.
  • One participant emphasizes that the impulse experienced by both the rock and the window is equal, but the outcome depends on the material strength of the window.

Areas of Agreement / Disagreement

Participants generally disagree on the interpretation of Newton's third law in the context of the rock and window interaction. While some assert that the law holds true, others question its applicability based on the observed behavior of the rock and the window.

Contextual Notes

Participants express uncertainty regarding the definitions of forces involved and the limitations of the window's material properties in exerting an equal and opposite force. The discussion highlights the complexity of interactions and the conditions under which Newton's third law applies.

  • #121


Archosaur said:
When will this thread die?

Good question, me-from-5-months-ago.

In fact, this question is even more pertinent today, considering that this mediocre thread has been sporadically active for... roughly 9% of my life.

*Sigh*... see you all in another 5 months.
 
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  • #122


satya98 said:
Is it the 3rd law


What the laws which we declared as universal are not applicable at all the portions of the universe. With out knowing at least 1% about the universe, how we can declare a law as universal.

How many laws are applicable at the center of the black hole?

You are being far too literal, and you are assuming the laws are applied literally too.

We know the context in which the laws hold, and we know that they do not hold everywhere. You don't need to go to the centre of a BH to find that.

Two objects moving toward each other do not simply add their velocities, they use relativistic velocity addition, even at human speeds. However, the effect is so vanishingly small that, unless we are dealing with objects moving at near relativistic velocities (or time spans where it's relevant, such as GPS navigation), we do not have to factor it in.

It does not mean we just throw the laws out the window.
 
  • #123


A Scientific Law is sort of defined to be an observation that will always occur when under specific conditions. So... we don't throw laws out of the window, we have a new set of them to use under different conditions.
 
  • #124


Newtype said:
I still don't understand. Momentum is basically force. Momentum equals mass multiplied by velocity, and force equals mass multiplied by acceleration (acceleration is the rate of change of velocity over time).

No, momentum is not force. The rate of change of momentum is force.

Suppose you had an object that is 10 kg and moving at 10 m/s.

It's momentum is (10 kg)(10 m/s) = 100 kg m/s.

Force is defined as the rate of change of momentum. This moving object is not accelerating, it's just moving at a constant velocity in a straight line. So what's its force?

F = ma = (10 kg)(0 m/s^2) = 0 N.

This object exerts no force, but has momentum of 100 kg m/s. Momentum and force are completely different.
 
  • #125


Some of you guys really crack me up. You seem to argue against some of the most fundamental ideas in basic Physics when you just haven't done your homework. Go back to basics and learn them. Only then should you feel qualified to 'question' stuff. This isn't a subjective subject. It's hard and as objective as it can possibly be.
 

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