Understanding Verlet Algorithm

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In summary: Yes, they are somewhat matching. Maybe you can increase the accuracy by using more sophisticated calculators or by doing the calculations yourself.
  • #1
Arman777
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I am trying to understand the verlet algorithm but I am kind of stuck.

1570303410061.png


I guess first we are findind the ##v(t + 1/2h)## then we are leaving it there and starting a loop for 8.78 ?

Also I did not understand the meaning of the equation 8.78 ? We are never using ##v(t + 3h/2)## ? Or in the loop ##v(t + 3h/2)## becomes ##v(t + 1/2h)##
 
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  • #2
Arman777 said:
I am trying to understand the verlet algorithm but I am kind of stuck.

View attachment 250751

I guess first we are findind the ##v(t + 1/2h)## then we are leaving it there and starting a loop for 8.78 ?
Yes, you calculate v(t + h/2) using the formula in 8.77.

No, there is no step 8.78, but there are steps 8.78a, 8.78b, and so on. And these steps aren't a loop -- you do them in succession. IOW, first you calculate r(t + h) as in 8.78a, then you calculate the vector k as in 8.78b, then v(t + h) and finally v(t + 3h/2).
Presumably there is some description of f(r(t), t) that isn't shown on the attached image.
Also part of the formula for step 8.78b is confusing in the 2nd argument to f. It looks like t + h, but maybe there is a smudge on the paper?
Arman777 said:
Also I did not understand the meaning of the equation 8.78 ? We are never using ##v(t + 3h/2)## ? Or in the loop ##v(t + 3h/2)## becomes ##v(t + 1/2h)##
It's not clear to me, either. It would be a good idea to look up Verlet algorithm to see if you can find a clearer description.
 
  • #3
Mark44 said:
Yes, you calculate v(t + h/2) using the formula in 8.77.

No, there is no step 8.78, but there are steps 8.78a, 8.78b, and so on. And these steps aren't a loop -- you do them in succession. IOW, first you calculate r(t + h) as in 8.78a, then you calculate the vector k as in 8.78b, then v(t + h) and finally v(t + 3h/2).
Presumably there is some description of f(r(t), t) that isn't shown on the attached image.
Also part of the formula for step 8.78b is confusing in the 2nd argument to f. It looks like t + h, but maybe there is a smudge on the paper?
It's not clear to me, either. It would be a good idea to look up Verlet algorithm to see if you can find a clearer description.
I searched online but they are using acceleration... I guess Verlet algorithm is also not much accurate... I did not understand the idea .. I was trying to solve a "one" body problem by using RK method but it seemed dead end so I was trying to do it with verlet but I couldn't understand the steps.

1570307551281.png


In Newtonian Gravity If I have $$d^2r/dt^2 = -GMr/r^3$$ so then in the above picture ##a(t)=-GMr/r^3## ?

I actually wanted to use Bulirsch-Stoer method but I am not sure how can I manage that without doing Verlet properly
 
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  • #4
Arman777 said:
In Newtonian Gravity If I have $$d^2r/dt^2 = -GMr/r^3$$ so then in the above picture ##a(t)=-GMr/r^3## ?
My copy of Halliday & Resnick shows one form of Newton's law of universal gravitation as
$$F = G\frac{m_1m_2}{r^2}$$
Your equation has just one mass in it -- M. For gravitational forces, you need to have two masses at least, with each attracting the other with forces that are equal in magnitude but opposite in direction.
 
  • #5
Mark44 said:
My copy of Halliday & Resnick shows one form of Newton's law of universal gravitation as
$$F = G\frac{m_1m_2}{r^2}$$
Your equation has just one mass in it -- M. For gravitational forces, you need to have two masses at least, with each attracting the other with forces that are equal in magnitude but opposite in direction.
$$F = m \frac{d^2\vec{r}} {dt^2} = -\frac{MmG\vec{r}} {r^3} $$
so
$$\frac{d^2\vec{r}} {dt^2} = -\frac{MG\vec{r}} {r^3}$$
 
  • #6
I guess I solved it ( The question claims that we should find a non circular orbit, but near elliptical one ). Here is the graph of the orbit
Figure_1.png


Now I ll try to calculate the total energy of the system if it matches with the real values great!
 
  • #7
Gotcha.
Arman777 said:
so then in the above picture ##a(t)=-GMr/r^3## ?
Looks fine to me. Keep in mind that r is a function of t. Also, in post #6, you're distinguishing between r in the numerator (a vector) and r in the denominator, the magnitude of the vector.
 
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  • #8
Mark44 said:
Gotcha.
Looks fine to me. Keep in mind that r is a function of t. Also, in post #6, you're distinguishing between r in the numerator (a vector) and r in the denominator, the magnitude of the vector.
Yes I was careful about that
Figure_1.png


"-." lines are potential energy
"--" lines are kinetic energy and other is total energy.

My findings
Potential energy: -5.30517849127762e+33 Kinetic Energy: 2.6525789562766106e+33 Total energy: -2.6525995467214404e+33

And these values from a site
-5.29201E+332.66762E+33-2.62439E+33
I guess they are somewhat matching. Maybe I can increase the accuracy by decreasing the time step (?)

Now I took the average of the energies, more close to the actual values. (Before this I only took the last values in the data array, :p what a mistake)
 
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  • #9
Arman777 said:
Maybe I can increase the accuracy by decreasing the time step (?)
Yes, up to a point. Beyond some level, you'll be getting errors in the calculations because computers don't do perfect floating point calculations.
 
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  • #10
Mark44 said:
Yes, up to a point. Beyond some level, you'll be getting errors in the calculations because computers don't do perfect floating point calculations.
Indeed. Also the method is not so perfect. Thanks for the help
 
  • #11
Arman777 said:
Now I took the average of the energies, more close to the actual values. (Before this I only took the last values in the data array, :p what a mistake)
Why are you doing this? There is nothing to be learned from looking at KE and PE separately and averaging them, the point is that the TOTAL energy should be CONSTANT at each step. The velocity verlet (and other leapfrog algorithms) are generally better at doing this than R-K methods because the error terms in the alternate 'half-steps' (8.78(a) and 8.78(c) in the extract you quoted in #3) tend to cancel out.

Why not go back to the R-K method and compare it side-by-side with the velocity Verlet to see how the total energy behaves? Also, run for 10, 100, 1000 orbits and see what happens.
 
  • #12
pbuk said:
Why are you doing this? There is nothing to be learned from looking at KE and PE separately and averaging them, the point is that the TOTAL energy should be CONSTANT at each step. The velocity verlet (and other leapfrog algorithms) are generally better at doing this than R-K methods because the error terms in the alternate 'half-steps' (8.78(a) and 8.78(c) in the extract you quoted in #3) tend to cancel out.

Why not go back to the R-K method and compare it side-by-side with the velocity Verlet to see how the total energy behaves? Also, run for 10, 100, 1000 orbits and see what happens.
I don't think energy will be conserved in the RK method. Well yes you are right, I just wanted to print them. For instance If the code runs for binary system and not for an normal orbit, we will need kinetic and potential energy values of the system. In this case avg values will not be usefull I guess but just for orbital case its good enough I guess.
 

1. What is the Verlet algorithm?

The Verlet algorithm is a numerical method used to solve equations of motion in classical mechanics. It is based on the idea of using position and velocity information at different points in time to approximate the motion of a system.

2. How does the Verlet algorithm work?

The Verlet algorithm works by using the position and velocity of a particle at two different points in time to estimate the position at a future time step. This is done by calculating the position at the next time step based on the current position and velocity, and then using that position to calculate the velocity at the next time step.

3. What are the advantages of using the Verlet algorithm?

One of the main advantages of the Verlet algorithm is that it is relatively simple to implement and can handle systems with complex interactions. It is also a symplectic integrator, meaning it conserves energy and momentum, which makes it useful for simulating physical systems.

4. What are the limitations of the Verlet algorithm?

The Verlet algorithm is not suitable for all types of systems. It is most effective for systems with conservative forces, meaning the total energy of the system remains constant. It is not as accurate as other numerical methods for solving equations of motion, and can introduce errors over long periods of time.

5. How is the Verlet algorithm used in scientific research?

The Verlet algorithm is used in a variety of scientific fields, including physics, chemistry, and biology. It is often used to simulate the behavior of molecules and atoms, as well as larger systems such as planets and galaxies. It is also used in computer graphics to simulate the movement of objects in video games and animations.

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