Re: Kinetic Energy formula check - a series of short burn rockets, and see how the velocity adds up.

From	Thomas Heger <ttt_heg@web.de>
Newsgroups	sci.physics, alt.sci.physics, sci.physics.relativity, uk.politics.misc
Subject	Re: Kinetic Energy formula check - a series of short burn rockets, and see how the velocity adds up.
Date	2023-01-12 08:31 +0100
Message-ID	<k29ra7F4a40U1@mid.individual.net> (permalink)
References	<tpe2q8$3qef8$1@dont-email.me> <k220anFsac9U1@mid.individual.net> <20230109141313.4df58ffe@jrenewsid.jretrading.com>

Cross-posted to 4 groups.

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Am 09.01.2023 um 15:13 schrieb Joe:
> On Mon, 09 Jan 2023 09:07:50 +0100
> Thomas Heger <ttt_heg@web.de> wrote:
>
>> Am 08.01.2023 um 10:36 schrieb Dave:
>>> Is it v or v^2?
>>>
>>> Is this formula E=0.5mv^2 correct, or is it more like E= mv?
>>
>>
>> That should depend on the used units for E, m and v.
>>
>> As unit systems have different degrees of internal consistency, you
>> should certainly be able to find an example for both factors.
>>
>>
>> I usually use SI units only and would get 0.5 as factor, but have not
>> verified the exponent 2 myself.
>>
>> In general I would prefer the quantity momentum over energy, because
>> v is frame dependent, hence also kinetic energy.
>>
>> Then I would like to compare conservation of momentum and
>> conservation of energy, but suggest choosing conservation of momentum
>> over conservation of energy.
>>
>> TH
>
> The whole question revolves around kinetic energy being a different
> thing from momentum, not just being a different word for it. To begin
> with, we have no idea of the absolute kinetic energy or the absolute
> momentum of an object, because we have no idea how fast an object is
> moving, and according Einstein, we never can know it. So we work on
> relative quantities, generally relative to 'stationary with respect to
> the bit of Earth's surface where the action takes place'.

Momentum and kinetic energy adress different things, but are closely 
related.

I chose as example a cannon ball flying through an empty 
gravitation-free SRT like space.

Now we don't know the velocity of the cannon ball, because we have no 
'anchor' to measure velocity against.

But that cannon ball shall hit a spaceship, which drifts around that 
void, too.

Now we have an anchor point, which is that spaceship and the point of 
its hull that the cannonball hits.

Now we see, that we need to take the relative velocity between ball and 
spaceship as velocity v and cannot use any other velocity.

In the case of collision between ball and ship the momentum of the ball 
is transferred to the ships armourment, as is also the kinetic energy of 
the ball.

But obviously relative velocity is the thing which matters, while 
absolute velocity wouldn't.

If we would use the SRT setting and would allow arbitrary velocities and 
arbitrary frames of references, then we would not have proportional 
relations between momentum and kinetic energy, because one behaves 
quatratic and one linear, if v is altered.

So, a different velocity caused by external changes of the position of 
observation would alter the relation between inertia and energy inside 
that cannon ball, what cannot possibly be the case. Therefore velocity v 
has to be the velocity in respect to the target, which is not arbitrary 
and not subject to decisions of an external observer.

> The kinetic energy of an object of mass m moving at velocity v is the
> amount of energy required to raise the mass from a stationary position
> to the velocity v, or to slow it down to stationary. You can start from
> first principles with length and time, going through force,
> acceleration and work, or you can work directly with velocity as the
> independent variable.

SRT does not know any 'stationary positions', because positions are 
always relative to something.

That somethings is also relative to something and so forth.

In the end we have a final point and still don't know, whether or not 
that point moves.

...

TH

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