Newton's first law?

[pjhirv]

What's not the point? The entire post is a discussion involving thinking about these things now. And getting a right answer before class starts.

Yes, you got it. Gongrats!

 

[sophiecentaur]

Prior to Newton, most people assumed that objects required a continuous force to keep them moving. And that the planets were kept in their orbits by the hand of God.

This belief persisted even to 1960 and beyond. In that year, the US launched a satellite which was a simple balloon and which stayed up in orbit for some while. I remember standing outside my grandad's cottage, along with my dad and we saw it ("the sateloon") go overhead. This was the very first satellite that was big enough actually to see - quite a big deal for me. My dear old grandad said "But why can't we hear the engine?". He blew it for me; up till then I thought he knew everything.

 

[Herman Trivilino]

Sputnik was seen by viewers on Earth.

 

[sophiecentaur]

Sputnik was seen by viewers on Earth.

OH? It was tiny but the satelloon was huge. Did you yourself see Sputnik? I wonder what the viewing figures were.

65 years ago the light pollution may have been less but I remember seeing it with no trouble. I may need to alter my statement to include the words "many many viewers in the general public" (at least some of my mates!). I remember many radio hams tuned into bleep bleep but the active radio source in sputnik was predictably detectable.

 

[PeroK]

This belief persisted even to 1960 and beyond. In that year, the US launched a satellite which was a simple balloon and which stayed up in orbit for some while. I remember standing outside my grandad's cottage, along with my dad and we saw it ("the sateloon") go overhead. This was the very first satellite that was big enough actually to see - quite a big deal for me. My dear old grandad said "But why can't we hear the engine?". He blew it for me; up till then I thought he knew everything.

By today's standards your grandad would have been progressive in not believing that the balloon was a government conspiracy to poison the atmosphere! Newton's first law is the least of it.

 

[Herman Trivilino]

Did you yourself see Sputnik?

I don't remember. I was 2 years old. :-)

 

[sophiecentaur]

I don't remember. I was 2 years old. :-)

I read in several articles (just google it) that sputnik1, at a distance of over 500km was not visible to the naked eye but, with the help of a radio receiver, could be found with binoculars. Sputnik 2 was a bit bigger and had a dog on board - same visibility. For those of a sensitive disposition I can report that one dog was, in fact, harmed in the experiment. There were stories that it was gassed humanely to avoid its suffering - but really??

 

[Herman Trivilino]

I read that near sunrise and sunset a glint from reflected sunlight could be seen.

 

[wrobel]

Strictly speaking the first Newton law says that inertial frames do exist.
By definition an inertial frame is the frame in which equations of a closed system's motion are invariant relative to the Galilean group.
(A mechanical system is said to be closed if it does not experience external influences)

 

[sophiecentaur]

I read that near sunrise and sunset a glint from reflected sunlight could be seen.

We can't say it never happened but the conditions would have been rare and fleeting and the viewers having elf-like eyesight. Bins and a radio helped to reveal Sputnik with a bit more certainty.
If that was the case then would we not see loads of space debris nowadays? We see many satellites easily these days but the reflecting areas involved are huge in comparison.

 

[Herman Trivilino]

We can't say it never happened but the conditions would have been rare and fleeting and the viewers having elf-like eyesight. Bins and a radio helped to reveal Sputnik with a bit more certainty.

Did you watch the movie October Sky? Don't remember for sure but think they witnessed it. Maybe it was fictionalized.

 

[sophiecentaur]

Did you watch the movie October Sky?

Did you ever see the film "The Sound Barrier"? It even made a claim for the wrong country and 'made up' a bit about reversing the controls.

 

[wrobel]

Some physics textbook writer told me that Newton's first law applies only on bodies

not bodies but particles

But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?

"No forces" and "zero forces" are the same thing.

 

[Herman Trivilino]

"No forces" and "zero forces" are the same thing.

Yes, but the OP's issue, as I understand it, is that "no forces" and "zero net force" are not the same.

 

[wrobel]

Yes, but the OP's issue, as I understand it, is that "no forces" and "zero net force" are not the same.

"No forces" and "zero net force" are in fact the same thing, since we are considering a single particle. If you have a particle at rest in an inertial frame, you can not distinguish one case from another