Evidence against SR: Rocket Motion

[martillo]

The real equations of motion rockets follows Classical Physics (F=ma) and not Relativity Theory (F=dp/dt) !!!

Please take a look at: Rocket Motion, Force and Momentum (http://www.geocities.com/anewlightinphysics/new_evidence/Rocket_Motion_and_Momentum.htm)

[Albers]

Our rockets don't go that fast. Aren't you using a special relativistic concept of mass?

[martillo]

No, is not relativistic but the equation dp/dt = m(dv/dt) + (dm/dt)v has a considerable component on the mass variation and F=ma and F=dp/dt will give different results.

[Albers]

How are you figuring and justifying mass change? (effective mass, relativistic mass, whatever you call it)

[martillo]

As I can note you havent got into thew problem yet.

A rocket has fuel and the fuel is loosed as it accelerates. The mass of a rocket includes its fuel. The varying mass is of the fuel.

[Albers]

I apologize, having thought you were speaking of relativistic effects here. What are you saying that you think people have not thought of? You are doing it correctly in a classical sense. Indeed......... f = d(mv)/dt, so expand: . .f=mdv/dt + vdm/dt , and rearrange: f - vdm/dt = mdv/dt . OK, now on the left-hand side are the original rocket force f, and a bonus, since change of mass in negative. We can even say the rocket force is dm/dt times some rocket efficiency R: f = -Rdm/dt describes our rocket; R is positive. Now, -(R+v)dm/dt = mdv/dt. Good, yah? What are the realistic ranges of these numbers? When does v become considerable to R? I just worked out a quick half-page showing a car moving 90 km/hr and burning about seven liters of gasoline in an hour. One must also know the power output and I estimated 50 Horsepower. I get v as 25 m/s, and R is about 30,000 times that. This is a very lossy machine.

[martillo]

Albers,
Indeed......... f = d(mv)/dt, so expand: . .f=mdv/dt + vdm/dt , and rearrange: f - vdm/dt = mdv/dt .
You cannot do that since the equation of motion of the rocket is derived from the principle of conservation of momentum applied to the composed body of rocket+fuel. dp/dt=0 for it and so you have the "thrust" equation of the rocket:
m(dv/dt) + u(dm/dt) = 0
or
m(dv/dt) = -u(dm/dt)
where u is the relative velocity between the fuel and the rocket.
This is the equation of the motion of the rocket and you cannot change it!

You can make a search on the web looking for "rocket motion equation" and see for example:
http://www.braeunig.us/space/propuls.htm
http://classicalmechanics.net/RocketMotion.htm

What are you saying that you think people have not thought of?
I believe I'm thinking different. I never heard about this before and I haven't found a similar reasoning on the web.

By the way a rocket with its fuel has a mass that varies considerably with its acceleration, a standard car doesn't.

[Albers]

Are we in different frames of reference distinguished by velocity and who is onboard? I see that you mention fuel vel. (I assume exhaust) but I am not looking at the local analysis you are.?.