# Recent content by DuckAmuck

1. ### I Momentum operator acting to the left

Right. I get that but I’m not sure where I go wrong here.
2. ### I Momentum operator acting to the left

Is the following true if the momentum operator changes the direction in which it acts? \langle \phi | p_\mu | \psi \rangle = -\langle \phi |\overleftarrow{p}_\mu| \psi \rangle My reasoning: \langle \phi | p_\mu | \psi \rangle = -i\hbar \langle \phi | \partial_\mu | \psi \rangle \langle...
3. ### A Separation of variables possible in this problem?

Is it possible to use separation of variables on this equation? au_{xx} + bu_{yy} + c u_{xy} = u + k Where u is a function of x and y, abck are constant. I tried the u(x,y) = X(x)Y(y) type of separation but I think something more clever is needed. Thank you.
4. ### I Elliptical Orbit and Kepler's equation

It is easy to find that the equation for an ellipse is: $$1 = x^2/a^2 + y^2/b^2$$ Then according to Kepler's equation: $$x = a(\cos(E)-e)$$ $$y = b\sin(E)$$ where E is the eccentric anomaly and e is the eccentricity. If you plug the Kepler's equations' x and y into the equation for the ellipse...

Thank you!
6. ### A Elliptical orbit parameterized by time

Yes. I get something non-analytic.
7. ### A Elliptical orbit parameterized by time

It is fairly trivial to do this with a circular orbit: $$(x,y) = (cos(\omega t),sin(\omega t))$$ where t is time, and $$\omega = \sqrt{GM/r^3}$$ How this parametric equation look for an elliptical orbit?
8. ### A Did NASA use something more efficient than a Hohmann Transfer?

$$t = \pi \sqrt{\frac{(r_1+r_2)^3}{8GM}}$$ $$r_1 + r_2 = 4e8 m$$ $$M = 6e24 kg$$ $$t=4.9 days$$
9. ### A Did NASA use something more efficient than a Hohmann Transfer?

I see. Free return trajectories are faster then? Since they have enough energy to return a craft with minimal/no burns? I’m just trying to account for the discrepancy in time between the Hohmann transfer time of 5 days and the actual 3 day time.
10. ### A Did NASA use something more efficient than a Hohmann Transfer?

Thanks. Are Bi-elliptic transfer orbits quicker than Hohmann transfers as well? Are they more akin to what the apollo missions used?
11. ### A Did NASA use something more efficient than a Hohmann Transfer?

It's common knowledge that it takes about 3 days to get to the moon. With a Hohmann transfer, I get a transit time of 5 days, not 3. I see NASA used something called "trans-lunar injection". Is this distinct from a Hohmann transfer, and more time efficient? What makes this trajectory different...
12. ### I Inelastic Collision Question

In collisions that are inelastic or partially elastic, how can we predict how much of the energy lost to the surroundings becomes heat, and how much becomes sound? What determines that fraction?
13. ### A Compactification of spatial extra dimensions

Hi everyone, I am looking at a paper on compact dimensions. Equation 65 makes sense except for the term of 4*pi*n*R in the denominator. Why is it 4*pi and not 2*pi? I cannot rationalize this. Please help. Thank you. https://arxiv.org/ftp/hep-ph/papers/0609/0609260.pdf
14. ### I Universe Expansion

So it is not yet known/understood?
15. ### I Universe Expansion

So the universe is expanding, and galaxies are getting farther apart from one another on average. Does this motion count the same as ordinary motion, in that if a galaxy is being expanded away from us at 0.5c, that clocks in that galaxy would appear to tick slower at 0.866 the rate of clocks here?