Angular momentum of earth from Lagrangian, am I correct?

In summary, the conversation discusses using Lagrangian mechanics to calculate the angular momentum of the Earth and the resulting values being off by two orders of magnitude. It is eventually discovered that the incorrect assumption of 15*10^6 meters for the Earth's distance from the Sun was made, leading to the error. The correct value of 15*10^10 meters is used to calculate the angular momentum and the final value is found to be 2.68395*10^40, which is consistent with previous calculations.
  • #1
genericusrnme
619
2
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
[tex]\mathcal{L} = \left(\frac{1}{2}m (r')^2 + \frac{1}{2}m r^2 (\theta ')^2\right)+\frac{G m M}{r}[/tex]
Applying the Euler Lagrange eqn to prove conservation of momentum conjugate to angle
[tex]0 = m r^2\theta '=L[/tex]
And solving for angular velocity
[tex]\theta '=\frac{L}{m r^2}[/tex]
Then applying the eqn to the radial component
[tex]m r (\theta ')^2-\frac{G m M}{r^2}= m a[/tex]
Assuming on a stable orbit net force on r should be zero and substituting in what I found for angular velocity
[tex]\frac{L^2}{m r^3}-\frac{G m M}{r^2}=0[/tex]
Then solving for L
[tex]L = \sqrt{G m^2M r}[/tex]
Plugging in values from wiki
[tex]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)}[/tex]
[tex]L = 2.68395*10^38[/tex]

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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  • #2
Further more I get
[tex]\theta '=\frac{L}{m r^2} [/tex]
[tex]\theta '\approx \frac{2.7\times 10^{39}}{\left(6\times 10^{24}\right)\left(1.5\times 10^9\right)^2}[/tex]
[tex]\theta '\approx 0.0002[/tex]
Which proves I'm 2 orders of magnitude off because I get
[tex]0.0002\ 60\ 60\ 24\ 365 = 6307.2[/tex]
which is about three orders of magnitude off of 2pi

where did my 1000 go? :(
 
Last edited:
  • #3
1 Astronomical Unit = 149 597 870 700 meters=15 *10^10 and not 15*10^6.
 

1. What is angular momentum of the earth?

The angular momentum of the earth is a measure of the rotational motion of the planet around its own axis. It is a vector quantity that takes into account both the mass and the velocity of the earth's rotation.

2. What is the Lagrangian of the earth?

The Lagrangian of the earth is a mathematical function that describes the kinetic and potential energy of the planet in terms of its position and velocity. It is used to calculate the equations of motion for the earth's rotation.

3. How is the angular momentum of the earth calculated from the Lagrangian?

The angular momentum of the earth can be calculated by taking the derivative of the Lagrangian with respect to the angular velocity of the earth. This equation is known as the Euler-Lagrange equation.

4. What factors affect the angular momentum of the earth?

The angular momentum of the earth can be affected by changes in its mass, its rotational speed, and its distance from other celestial bodies. It can also be influenced by external forces such as gravitational pull from other planets.

5. Is the angular momentum of the earth constant?

No, the angular momentum of the earth is not constant. It can change over time due to external forces and interactions with other celestial bodies. However, the total angular momentum of the earth and its orbit around the sun remains constant due to the conservation of angular momentum.

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