XPost: sci.physics, alt.sci.physics
On 22 09, Dave wrote:
On 22 12, Dave wrote:
On 22 40, Dave wrote:
On 22 10, Dave wrote:
This is really one for aspiring physics teachers.
There are issues with kinetic energy between a rocket,
and a gravitational drop both in a vacuum.
Gravitational drop:
gravity is 10ms-2. i.e. every second the velocity increases by 10m/s
Say mass = 20 kg
Kinetic energy is taught as E=1/2m v^2.
Height needed to get to the energy with E=mgh, h=E/(mg),
conservation of energy
Also works out with speed and distance, s=ut + 1/2at^2
Time(s) Speed(m/s) Kinetic Energy Height Needed (m)
1 10 1000 J 5 >>>> 2 20 4000 J 20 >>>> 3 30 9000 J 45 >>>> 4 40 16000 J 80 >>>> 5 50 25000 J 125 >>>>
This is basic secondary school physics
Now with a rocket in a vacuum in space, or on a frictionless sled.
No wheels so no complexity from rotational inertia.
Using F=ma, Newtons law, a 20kg mass, and a thrust of 200N, to get
an acceleration of 10m/s. It so happens that 200N is a standard size
for a little rocket engine. Uses about 150g of fuel (mostly
oxidiser) per second.
150g on 20kg shouldn't affect the calculations too much.
However there are interesting things when the rocket is given an
initial
velocity. A one second burn uses the same energy (mass of fuel), and
should increase the speed so the kinetic energy by the same amount -
not near light speed.
To get to the initial speed is beside the point, could use a big
rubber band.
KE- kinetic energy, 1/2 m v^2.
M - 20KG
Initial Speed after Initial KE Final KE KE increase
speed (m/s) burn(m/s)
0 10 0J 1000J 1000J
Initial Speed after Initial KE Final KE KE increase
speed (m/s) burn(m/s)
10 20 1000J 4000J 3000J
Initial Speed after Initial KE Final KE KE increase
speed (m/s) burn(m/s)
50 60 25000J 36000J 11000J
Initial Speed after Initial KE Final KE KE increase
speed (m/s) burn(m/s)
1000 1010 10,000,000J 10,201,000J 201,000J >>>>
The same burn gives a completely different increase in kinetic
energy, makes no sense presented like this.
So this is really one for aspiring physics teachers. What do you
say to a smart kid who challenges you with this?
A- shut up and stop annoying me
B- give the correct physics
With A you've lost a student, and they will go and do something else.
Problems are from
1- my understanding of what is taught
or
2- what is being taught
Regarding 1- this is well below light speed, so relativistic
refinements
shouldn't apply, and the drop is all quite near the earth, so gravity
can be seen as constant, over say several hundred meters
Regarding 2- there are good reasons why the would want to teach
an incorrect information, to keep the enemy stupid. Unfortunately
what was good in about year 1903, is now holding us back. The "lie" has >>>> become the accepted "truth".
Would love a "correct" physics, since what is being taught is likely
what is keeping people back from making more advanced aerial craft.
Current conclusion after some chat:
Classical physics doesn't describe the real universe very well, but
an idealised one. It isn't even self consistent at the human scale
(new insight thanks to rocket sled thought experiment).
I am willing to change on new information given.
Specifically problems exist with kinetic energy and momentum. Kinetic
energy comes from the conservation of energy (conversion of
gravitational potential energy to kinetic energy) and acceleration in
free fall at the same rate.
Am I bothered - no I'm retired. Others might be. Also in no good
conscience could I teach the classical mechanics, since I'm not
coming across eternal truths. If I were hard up, yes, I would
regurgitate for pay.
Makes more sense with the change in speed proportional to the distance
dropped, not the time falling. Why would the acceleration depend on
the initial speed?
This would keep a consistency with the conservation of energy in a
good way, the rocket sled, and momentum and kinetic energy are
harmonised.
Can't think of everything all at once, and thanks to Jim, the thread
has been kept alive for this information to arrive (after good sleep).
Might also explain why the only British lander on Mars crashed badly.
Hope to give updated numbers very soon.
i.e. Have acceleration due to gravity in a drop as velocity increase per meter dropped.
It might be that everything is still consistent with accepted basics,
don't know at this stage. Acceleration as velocity increase per unit
time is so engrained it might take a while to move on from this.
Energy to move stuff up is E=mgh. This is self evident, irrespective of units. Moving heavy stuff up takes more effort, and moving it up further takes more effort. Seems linear for both variables close to the planet.
For the drop, assume that freefall acceleration starts and measures at
9.8ms^2, very successfully and in vacuum chambers in various countries
for 100s of years.
s=ut + 1/2 at^2
v=u+at
So in 1 second s= 0.5x10 * 1*1 = 5m fall in a second
So speed increases to 10m/s in 1 second
Speed increase rate is 2m/s per meter.
The speed increases with distance, not time. (Cassandra Physics)
For conservation of energy with E=mgh,
Air resistance is ignored 20kg mass.
Input energy is mgh, but not the regular g, but G (from Cassandra) at 2
Height Speed Input energy(mGh) Kinetic Energy (m*v)
G is 2
5 10 200 (20*2*5) 200
10 20 400 (20*2*10) 400
20 40 800 800
30 60 1200 1200
40 80 1600 1600
50 100 2000 2000
60 120 2400 2400
70 140 2800 2800
100 200 4000 (20*2*100) 4000
150 300 6000 (20*2*150) 6000
200 400 8000 (20*2*200) 8000
1000 2000 40000 (20*2*1000) 40000
So there you have it, all self consistent.
If I'm wrong I'll post a grovelling apology
(concept, not the sums, regular apology for bad arithmetic).
If they have been teaching the wrong physics, there is a need a great
purging of professors
Either:
1- they have been teaching wrong facts knowing what is correct,
thus confusing people and bringing education into disrepute - why
believe them on climate change, guilt by association is only human
or
2- they should have worked out the basics and should be fired
for stupidity (ignorant repeater).
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