Re: Using nuclear power to make renewables and a hydrogen economy cost effective
From: Ian St. John (istjohn_at_noemail.usa)
Date: 10/30/04
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Date: Sat, 30 Oct 2004 19:41:27 -0400
Hatunen wrote:
> On Fri, 29 Oct 2004 17:04:56 -0400, "Ian St. John"
> <istjohn@noemail.usa> wrote:
>
>> Hatunen wrote:
>>> On Fri, 29 Oct 2004 15:31:52 -0400, "Ian St. John"
>>> <istjohn@noemail.usa> wrote:
>>>
>>>> G. R. L. Cowan wrote:
>>>>> "Ian St. John" included:
>>>>>>
>>>>>> Actually, the energy is about half in the rocket and half in the
>>>>>> rocket exhaust. It is called the 'equal and opposite reaction'
>>>>>> thing.
>>>>>
>>>>> Equal and opposite momentum, not energy.
>>>>
>>>> Momentum is the resistance to change in motion. Mass times
>>>> velocity. Really just the measure of mass (inertia) of a moving
>>>> object.
>>>>
>>>> Newtons third law relates to forces. "for every applied force,
>>>> there is an equal and opposite reaction force".
>>>>
>>>> The forces are equal. The time of application (passage of the
>>>> exhaust through the rocket motor) is equal. The velocities and
>>>> masses are, of course, different.
>>>>
>>>> You are probably confusing the conversion efficiency of a rocket,
>>>> from potential to kenetic ( efficiency of fuel use), which is
>>>> indeed based on momentum. This does not mean that the two objects
>>>> ( the rocket and the exhaust ) do not equally share the energy
>>>> that is converted.
>>>>
>>>> http://nov55.com/roc.html
>>>> "Since the 4kg mass had twice as much momentum, the result
>>>> demonstrates that the fuel transformed in proportion to the
>>>> momentum rather than the ½mv² ( energy)."
>>>
>>> You said, "Actually, the energy is about half in the rocket and
>>> half in the rocket exhaust." That is not true. It is the momentum
>>> of the exhaust and the rocket that are equal and opposite
>>> relative to the center of mass of both before that bit of fuel
>>> was exhausted.
>>>
>>> A curious fact needs to be noted: if a rocket at rest in free
>>> space is fired it will move off, but the center of mass of all
>>> the fuel exhausted plus the rocket itself remains in the same
>>> place.
>>
>> And how does this dispute what I said?
>>
>>
>> The energy goes into creating a force and the force is shared equally
>> between the exhaust and the rocket accordiing to Newtons third law.
>
> I'm afraid I haven't the patience to explain what's wrong with
> that statement. I suggest a good college level first year course
> in physics.
Well, I have tried to explain some simple concepts and have failed. I don't
know whether to complaim about my own performance as a teacher or yours as a
student. I expect that you somehow thing it is the other way around anyway.
>
>> One can
>> also look at momentum and center of mass but I don't see how you
>> apply it to dispute the equal and opposite reaction forces.
>
> Forces are not energy. And energy is not required to create
> forces.
In this case it is. The force arises from the temperature increase and
subsequent expansion of the exhaust products. Creating that force is the
whole rationale for rocket ( reaction ) motors.
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