Angular momentum of earth from Lagrangian, am I correct?

AI Thread Summary
The discussion focuses on calculating the angular momentum of the Earth using Lagrangian mechanics. The Lagrangian is defined, and the Euler-Lagrange equation is applied to derive expressions for angular momentum and angular velocity. The user finds an angular momentum value of L = 2.68395*10^38 but suspects it is two orders of magnitude lower than expected. The error is identified as a miscalculation in the distance, confusing astronomical units. The user concludes that correcting the unit leads to a significant adjustment in the results.
genericusrnme
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Hey, I was just playing about with some lagrangian mechanics and tried to work out the angular momentum of the earth;
Starting with the Lagrangian
\mathcal{L} = \left(\frac{1}{2}m (r')^2 + \frac{1}{2}m r^2 (\theta ')^2\right)+\frac{G m M}{r}
Applying the Euler Lagrange eqn to prove conservation of momentum conjugate to angle
0 = m r^2\theta '=L
And solving for angular velocity
\theta '=\frac{L}{m r^2}
Then applying the eqn to the radial component
m r (\theta ')^2-\frac{G m M}{r^2}= m a
Assuming on a stable orbit net force on r should be zero and substituting in what I found for angular velocity
\frac{L^2}{m r^3}-\frac{G m M}{r^2}=0
Then solving for L
L = \sqrt{G m^2M r}
Plugging in values from wiki
L = \sqrt{\left(6.67\times 10^{-11}\right)\left(6\times 10^{24}\right)^2\left(2\times 10^{30}\right)\left(15\times 10^6\right)}
L = 2.68395*10^38

But I think I'm two orders of magnitude off, I remember it being to the power 40 not 38
Have I done something wrong here or made any incorrect assumptions?
 
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Further more I get
\theta '=\frac{L}{m r^2}
\theta '\approx \frac{2.7\times 10^{39}}{\left(6\times 10^{24}\right)\left(1.5\times 10^9\right)^2}
\theta '\approx 0.0002
Which proves I'm 2 orders of magnitude off because I get
0.0002\ 60\ 60\ 24\ 365 = 6307.2
which is about three orders of magnitude off of 2pi

where did my 1000 go? :(
 
Last edited:
1 Astronomical Unit = 149 597 870 700 meters=15 *10^10 and not 15*10^6.
 
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