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

In summary, Newton's first law of motion states that an object in motion will remain in motion unless acted upon by an external force. However, this law depends on the frame of reference in which it is observed. In an accelerating frame of reference, such as a rocket in deep space, an object may appear to accelerate without any external force acting on it. This is because the acceleration of the frame of reference is also affecting the object. Similarly, in a rotating frame of reference, inertial forces such as the centrifugal force and the Coriolis force must be considered. Overall, Newton's first law of motion applies differently in different frames of reference.
  • #1
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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  • #2
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".
 
  • #3
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.
 
  • #4
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".
 
  • #5
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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  • #6
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.
 
  • #7
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.
 
  • #8
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.
 

1. What is a frame of reference in the context of Newton's first law?

A frame of reference is a set of coordinate axes used to describe the motion of an object. It is a relative reference point from which the position, velocity, and acceleration of an object can be measured.

2. How does Newton's first law apply to different frames of reference?

Newton's first law states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. This law applies to all frames of reference, regardless of their motion or orientation.

3. Are there any exceptions to Newton's first law in certain frames of reference?

No, Newton's first law is a fundamental principle of motion and is universally valid, regardless of the frame of reference. However, it may seem to not hold true in certain frames of reference due to the presence of external forces or non-inertial forces.

4. How does the concept of inertia relate to frames of reference in Newton's first law?

Inertia is the tendency of an object to resist changes in its state of motion. In the context of Newton's first law, the concept of inertia is closely related to frames of reference because it is the basis for the law. In a frame of reference where there are no external forces acting on an object, it will remain at rest or in motion due to its inertia.

5. Can Newton's first law be used in non-inertial frames of reference?

Yes, Newton's first law can be used in non-inertial frames of reference, but it may require additional considerations. In non-inertial frames, there may be forces acting on an object that cause it to accelerate, making the law seem invalid. However, these forces are due to the motion of the frame itself and can be accounted for in the equations of motion.

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