Frame of reference in which Newton's first law is not valid

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Discussion Overview

The discussion revolves around the conditions under which Newton's first law of motion may not hold true, particularly focusing on non-inertial frames of reference. Participants explore examples and theoretical implications related to acceleration and force in these frames.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that in an accelerating frame of reference, such as a rocket in deep space, objects can appear to accelerate without an apparent force acting on them.
  • One participant suggests that if inertial forces are included in a non-inertial frame, then Newton's laws still apply, implying a distinction between inertial and non-inertial frames.
  • Another participant raises a question about the role of gravitational forces in defining these frames, particularly in the context of deep space.
  • There is a discussion about the equivalence of experiences in different frames, such as sitting at a desk versus being in a rocket, and how gravity can be perceived differently.
  • One participant asserts that when a pencil is dropped in an accelerating rocket, it does not accelerate in the same way as it would in an inertial frame, leading to a clarification that both perspectives can be valid depending on the frame of reference considered.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of acceleration and force in non-inertial frames, with no consensus reached on the implications of these examples. The discussion remains unresolved regarding the nuances of how Newton's laws apply in various contexts.

Contextual Notes

Participants highlight the complexity of defining forces and accelerations in non-inertial frames, indicating that assumptions about gravitational effects and inertial forces may influence interpretations.

parshyaa
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Newtons first law of motion depends on frame of reference
  • So what are some examples , which shows that in this frame of reference F = 0, but a is not equal to zero or vice versa.
 
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parshyaa said:
Newtons first law of motion depends on frame of reference
  • So what are some examples , which shows that in this frame of reference F = 0, but a is not equal to zero or vice versa.
Suppose that you are on a rocket out in deep space. The engines are firing so that the craft experiences one gee of acceleration. You hold out your hand and drop a pencil which, from your accelerating point of view, falls and hits the floor. There is no force on the pencil, yet it accelerates. This is a case of an "accelerating frame of reference".
 
parshyaa said:
Newtons first law of motion depends on frame of reference
  • So what are some examples , which shows that in this frame of reference F = 0, but a is not equal to zero or vice versa.

If F includes the inertial forces in an non-inertial frame, then Newtons 1st and 2nd still apply (that's what inertial forces are introduced for). Otherwise any non-inertial frame is an example where they don't apply.
 
Thank you so much. I wonder you said "deep space" so that gravitational force is 0.
jbriggs444 said:
Suppose that you are on a rocket out in deep space. The engines are firing so that the craft experiences one gee of acceleration. You hold out your hand and drop a pencil which, from your accelerating point of view, drops and hits the floor. There is no force on the pencil, yet it accelerates. This is a case of an "accelerating frame of reference".
jbriggs444 said:
Suppose that you are on a rocket out in deep space. The engines are firing so that the craft experiences one gee of acceleration. You hold out your hand and drop a pencil which, from your accelerating point of view, falls and hits the floor. There is no force on the pencil, yet it accelerates. This is a case of an "accelerating frame of reference".
 
parshyaa said:
Thank you so much. I wonder you said "deep space" so that gravitational force is 0.
Yes, that is why I put you in a rocket rather than in a chair sitting at your desk.

As A.T. hints, there is a sense in which sitting at your desk and sitting in a rocket are equivalent. Gravity need not be regarded as a force but merely a result of the fact that we choose to consider the surface of the Earth to be continuously at rest. But I did not want to complicate a simple question with a subtle answer.
 
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The other most common non-inertial frame is a rotating frame. An inertial particle will accelerate, leading to the use of the centrifugal force and the Coriolis force to describe the motion.
 
jbriggs444 said:
Suppose that you are on a rocket out in deep space. The engines are firing so that the craft experiences one gee of acceleration. You hold out your hand and drop a pencil which, from your accelerating point of view, falls and hits the floor. There is no force on the pencil, yet it accelerates. This is a case of an "accelerating frame of reference".
now wait a min. if you are in a rocket and drop that pencil, it doesn't accelerate anymore .. it falls to the rear of the rocket and sticks to the rear wall with 1g of force.
 
zanick said:
now wait a min. if you are in a rocket and drop that pencil, it doesn't accelerate anymore .. it falls to the rear of the rocket and sticks to the rear wall with 1g of force.
It does accelerate -- as measured against the accelerating frame of reference in which the rocket is motionless.
It does not accelerate -- as measured against an inertial frame in which the pencil is motionless.

Both are true.
 

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