Re: Newton's second law?

From: Androcles (dummy_at_dummy.net)
Date: 12/20/04 

Date: Mon, 20 Dec 2004 04:14:30 GMT

"Andr? Michaud" <srp@microtec.net> wrote in message
news:562f286c.0412191821.80b9cba@posting.google.com...
> Androcles a écrit :
>>
>> "Andr? Michaud" <srp@microtec.net> wrote in message
>> news:562f286c.0412181513.2828d241@posting.google.com...
>> > Mike a écrit :
>> >>
>> >> François Guillet wrote:
>> >> > If mass is time depending, is it F=m(t)*dv/dt or F=d(m(t)*v)/dt?
>> >> >
>> >> > FG
>> >>
>> >> F = dp/dt,p = mv
>> >>
>> >> In general: F = mdv/dt + vdm/dt (rocket equation)
>> >>
>> >> If m is constant it reduces to F = mdv/dt
>> >>
>> >> By the way you should know that Newton's second law is F=dp/dt as
>> >> stated in Principia and not F=ma, a result of popular ignorance.
>> >>
>> >
>> > Since t = d/v,
>>
>> True
>>
>> both forms of the equation are exactly equivalent,
>>
>> False.
>>
>> > so F=ma is perfectly legit
>>
>> False.
>> It was already explained to you above.
>
> I don't think so.
>
>> You are assuming m is constant.
>
> That's precisely what Newton based his 2nd law on.
>
>> In the case of a rocket, it is not.
>> Something as simple as that is usually said to be "not rocket
>> science",
>> but in this case it is :)
>> Androcles
>
> To my knowledge, Newton was addressing the question of motion of
> stable masses, not the question of rocket launching, this is why
> Mike's comment was off the mark.

Gunpowder and the cannon's recoil were known to Newton, as were signal
rockets. That is why Mike is correct and your knowledge is deficient.
Mike said:
quote:
 In general: F = mdv/dt + vdm/dt (rocket equation)
 If m is constant it reduces to F = mdv/dt
unquote.
Note the "If m is constant".

> Rocket launching is a special case.

Not at all. It is included in F = dp/dt.

> Universal motion of stable
> masses is the general case for which the 2nd law was established
> by Newton.

You are guessing.

>
> The equation that applies to rockets is a simple extension of
> the basic 2nd law equation for stable masses.
>
Not at all. The second law is F = dp/dt.
F= ma is taught in schools as a precursor to aid understanding.
It is time for you to progress beyond that.
Androcles.

> André Michaud

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