Understanding Newton's First Law of Motion in Physics

[Meson080]

The below sentence has been extracted from the book-Concepts of Physics 1-H C Verma:

Newton's first law of motion: If the (vector) sum of all the forces acting on a particle is zero then and only then the particle remains unaccelerated (i.e, remains at rest or moves with constant velocity).

Won't the sentence be complete if its mentioned, with respect whom the particle remains at rest or moves with constant velocity?

 

[Doc Al]

Won't the sentence be complete if its mentioned, with respect whom the particle remains at rest or moves with constant velocity?

Any inertial frame will do.

 

[phinds]

Won't the sentence be complete if its mentioned, with respect whom the particle remains at rest or moves with constant velocity?

Yes, you are correct. The statements "remains at rest" and "moves with constant velocity" are meaningless without a frame of reference.

 

[A.T.]

Any inertial frame will do.

And if "all the forces" includes inertial-forces, non-inertial frames work too.

 

[Meson080]

Newton's first law of motion: If the (vector) sum of all the forces acting on a particle is zero then and only then the particle remains unaccelerated (i.e, remains at rest or moves with constant velocity) with respect to an inertial frame.

I hope the above statement makes sense, does it?

Does the Newtons first law hold good for particles "photons"?

 

[Doc Al]

I hope the above statement makes sense, does it?

Yes, makes sense to me.

Does the Newtons first law hold good for particles "photons"?

Photons are always moving at speed C with respect to a local frame. Newton's laws do not apply to photons. At high speeds, Newton's laws must be replaced with special relativity.

 

[Meson080]

Photons are always moving at speed C with respect to a local frame. Newton's laws do not apply to photons. At high speeds,Newton's laws must be replaced with special relativity.

I am not happy with just one letter "s" in your word Newton's law"s". It made sense to me to say that Newton's second law doesn't hold good for higher speeds, because of not considering mass to be constant. But, why doesn't Newton's first and third law hold good for higher speeds?

 

[sagarbhathwar]

In the Book "Concepts of physics by HC Verma", those kind of problems are mentioned in a fixed inertial frame unless mentioned otherwise. Simple.

 

[Doc Al]

I am not happy with just one letter "s" in your word Newton's law"s". It made sense to me to say that Newton's second law doesn't hold good for higher speeds, because of not considering mass to be constant. But, why doesn't Newton's first and third law hold good for higher speeds?

Newton's first law is fine, even in relativity. Newton's third law has problems, even non-relativistically. (Consider electromagnetic forces.) But when generalized to conservation of momentum, it's fine.